A  MANUAL 


OF 


FIRE   ASSAYING 


BY 

CHARLES    HERMAN    FULTON,   E.M, 

PRESIDENT    AND    PROFESSOR    OF    METALLURGY 
IN    THE     SOUTH     DAKOTA    SCHOOL     OF     MINES 


1907 
HILL    PUBLISHING    COMPANY 

505   PEARL  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

The  Engineering  and  Mining  Journal  —  Power  —  American  Machinist 


'  /v 


F  7  7 


Copyright,  1907,  BY  THE  HILL  PUBLISHING  COMPANY, 

ALSO  ENTERED  AT  STATIONERS'  HALL,  LONDON,  ENGLAND 

All  rights  reserved 


Hill  Publishing  Company,  New  York,  U.S.A. 


THIS  BOOK  IS  LOVINGLY  DEDICATED  BY 
THE  AUTHOR 


PREFACE 

THE  author  has  long  recognized  the  need  of  a  work  on  fire 
assaying  that  treats  the  subject  from  the  scientific  and  rational 
point  of  view  rather  than  from  that  of  the  "rule  of  thumb/' 
Strangely  enough,  this  last  governs  most  modern  works  on  the 
subject.  The  book  is  closely  confined  to  the  subject  of  fire 
assaying,  which  it  treats  in  detail.  The  chapters  on  "Reduction 
and  Oxidation  Reactions,"  "Crucible  Assay  and  Assay  Slags," 
and  "Cupellation,"  outline  scientifically  the  principles  of  assaying. 
A  large  part  of  these  chapters  is  new  and  some  of  the  material  is 
presented  for  the  first  time.  The  chapter  on  the  "  Errors  in  the 
Assay  for  Gold  and  Silver  "  outlines  and  discusses  the  accuracy 
of  the  assay  in  greater  detail  than  has  been  attempted  heretofore. 

The  author  has  had  experience  with  practically  all  of  the 
methods  of  assay  discussed  in  the  book;  first  as  a  manipulator, 
then  as  a  teacher,  and  finally  in  charge  of  works.  The  book  is 
intended  for  the  use  of  students  in  technical  schools  and  for  the 
assayer  in  actual  daily  practice  who  frequently  feels  the  need  of 
a  reference  book. 

The  author  wishes  to  acknowledge  his  indebtedness  to  the 
writers  cited  in  the  text,  especially  to  the  late  Professor  E.  H. 
Miller,  of  Columbia  University,  whose  work  and  personality  has 
ever  been  an  inspiration  to  the  author.  He  also  expresses  his 
thanks  to  Mr.  J.  B.  Read  and  Mr.  Ivan  E.  Goodner,  chemist  and 
assayer  respectively  for  the  Standard  Smelting  Company,  Rapid 
City,  and  to  Mr.  Frank  Bryant,  his  assistant  at  the  School  of 
Mines,  for  valuable  aid  in  the  testing  of  methods;  to  Professor 
M.  F.  Coolbaugh  for  the  inspection  of  those  chapters  containing 
chemical  equations,  etc.,  and  to  Miss  Ethel  Spayde  and  Miss 
Delia  M.  Haft  for  invaluable  aid  received  in  the  preparation  of 
the  manuscript  for  publication.  The  author  also  desires  to 
express  his  appreciation  of  the  courtesy  of  the  Denver  Fire  Clay 
Company  and  of  Ainsworth  &  Son,  Denver,  Colorado,  of  F.  W. 


vi  PREFACE 

Braun  and  Company,  Los  Angeles,  California,  and  others,  in 
furnishing  photographs  and  electrotypes  of  apparatus  used  in 
the  book. 

CHARLES  HERMAN  FULTON. 

RAPID  CITY,  S.  D. 
April,  1907. 


CONTENTS 

PAGES 

PREFACE v,  vi 

CHAPTER   I 

ASSAY  FURNACES  AND  TOOLS        1-19 

Assay  Furnaces.  Fuels.  Coal-burning  Muffle-Furnaces.  Con- 
struction. Dimensions.  Wood-burning  Furnace.  Coke  Furnace. 
Gasolene  Furnace.  Gas  Furnace.  Capacities  of  Furnaces  and 
Cost  per  Assay  for  Fuel  with  Different  Furnaces.  Muffles. 
Furnace  Tools,  Tongs,  etc.  Multiple  Scorifier  Tongs.  Cupel 
Charging  Device.  Molds.  Crucibles.  Scorifiers,  Roasting  Dishes, 
etc.  Crushing  and  Pulverizing  Apparatus. 

CHAPTER   II 

DEFINITIONS;  REAGENTS;  THE  ASSAY  OF  REAGENTS 20-27 

Definitions  of  Assaying.  General  Method  for  the  Determination 
of  Gold  and  Silver.  Reagents  Used  in  Assaying.  Assay  of  Reagents. 

CHAPTER   III 

SAMPLING 28-33 

Methods  of  Sampling.  Classification  of  Sampling  Methods. 
Principles  of  Sampling.  Sampling  in  Mills  and  Smelters.  Coning 
and  Quartering.  Sampling  by  Alternate  Shovel.  Sampling  Appa- 
ratus in  the  Assay  Laboratory.  Preparation  of  the  Assay  Sample. 
Control  Assays.  Umpire  Assays.  Mode  of  Settlement.  Sampling 
Lead  and  Copper  Bullion. 

CHAPTER   IV 

WEIGHING;  BALANCES  AND  WEIGHTS 34-42 

The  Assay  Balance.  Method  of  Setting  Up.  Construction.  Dis- 
cussion of  the  Principles  of  the  Balance.  Sensibility.  Weighing. 
Determination  of  Length  of  Balance-arms.  Pulp  Balance.  Weights. 
The  Assay-ton  System. 

vii 


viii  CONTENTS 

\ 

CHAPTER  V 

PAGES 

REDUCTION  AND  OXIDATION  REACTIONS 43-53 

Reduction.  Oxidation.  Reduction  of  Lead  from  Litharge  by 
•  Argol  by  Carbon,  by  Flour,  by  Sulphides.  Influence  of  Soda.  Re- 
duction of  Lead  from  Lead  Silicates.  Oxidation  of  Impurities  by 
Niter.  Niter  Reactions  with  Reducing  Agents.  Reaction  between 
Metallic  Lead  and  Niter.  Niter  and  Carbon.  Niter  and  Pyrite. 
Influence  of  Silica  on  the  Reactions.  Charges  to  Determine 
Reducing  and  Oxidizing  Powers.  Oxidizing  Power  in  Ores. 

CHAPTER   VI 

THE  CRUCIBLE  ASSAY;     ASSAY  SLAGS 54-66 

Nature  of  the  Crucible  Assay.  Influence  of  Fineness  of  Crushing. 
Mode  of  Occurrence  of  Gold  and  Silver,  Physical  Properties  of 
the  Slag,  Chemical  Properties  of  the  Slag.  Formation  Temper- 
atures of  Assay  Slags.  Nature  of  Assay  Slags  and  their  Mode 
of  Formation.  Influence  on  the  Assay  of  the  Formation  Temper- 
ature. Constituents  of  Assay  Slags.  Classification  by  Silicate 
Degree.  Influence  of  Silicate  Degree  and  Different  Bases  on  For- 
mation Temperature.  Eutectic  Compositions  of  Slags.  Table  of 
Assay  Slags.  Table  for  the  Calculation  of  Slags.  Example  of 
the  Calculation  of  an  Assay  Slag.  Composition  of  Ores.  Assay 
Slags  Commonly  Made.  Color  of  Slags. 

CHAPTER   VII 

CUPELLATION 67-80 

Bone-ash.  Composition,  Purification.  Making  Cupels.  Pro- 
cess of  Cupellation.  Sprouting  of  Beads.  Freezing-point  Curves  of 
Lead-Silver  and  Lead-Copper.  Temperature  of  Cupellation.  In- 
fluence of  Impurities  in  the  Lead  Button.  Influence  of  Copper. 
Of  Tellurium.  Color-scale  of  Temperature. 

CHAPTER   VIII 

PARTING 81-83 

Ratio  of  Gold  to  Silver  Necessary  to  Part  with  Nitric  Acid. 
Inquartation.  Strength  of  Acid.  Temperature  of  Acid.  Parting 
Devices.  Annealing. 

CHAPTER   IX 

THE  ASSAY  OF  ORES  CONTAINING  IMPURITIES         ^84-102 

Definition  of  Impurities.  Common  Impurities.  Effect  of  Im- 
purities on  Assay.  Production  of  a  Matte.  Effect  of  Silica.  Effect 
of  Soda.  Kind  of  Impurities.  Standard  Methods  of  Assay. 
Roasting.  Niter  Method.  Miller's  Oxide-Slag  Method.  Perkins' 


CONTENTS  ix 

PAGES 

Excess-litharge  Method.  Niter-Iron  Method.  Nature  of  the  Iron- 
Nail  Fusion.  The  Cyanide  Method.  Comparison  of  the  Different 
Crucible  Methods  of  Assay.  Scorincation.  Scorifiers.  Amount  of 
Lead  Used.  Amount  of  Ore.  Process  of  Scorincation.  Tem- 
perature of  Scorincation.  Rescorification.  Order  of  Oxidation  of 
Metals.  Applicability  of  Scorincation.  Combination  Method.  For 
Blister  Copper.  For  Mattes.  For  Cyanide  Precipitates.  Pre- 
cautions to  be  Observed  in  the  Method. 

CHAPTER   X 

SPECIAL  METHODS  OF  ASSAY 103-119 

Telluride  Ores.  Assay  by  Cripple  Creek  Flux.  By  Excess- 
litharge  Flux.  Behavior  of  Tellurium  in  the  Fusion.  Amount 
of  Tellurium  Present  in  Ores.  Losses  Caused  by  Tellurium. 
Assay  of  Complex  Tellurides  with  Different  Fluxes.  Results  Ob- 
tained. Assay  of  Copper-bearing  Material.  By  Scorincation.  Re- 
sults Obtained.  Excess -litharge  Method  for  Blister  Copper.  By 
Crucible  Assay.  Assay  of  Material  Containing  Zinc.  Effect  of 
Zinc.  Scorincation  for  Zinc  Ores.  Crucible  Fusion  for  Zinc  Ores. 
For  Cyanide  Precipitates.  Assay  of  Material  Containing  Graphite. 
Assay  of  Antimonial  Gold-Silver  Ores.  Arsenical  Ores.  Difficulties 
Experienced  with  these  Ores.  Assay  of  Heavy  Sulphides.  Assay 
of  Material  Containing  Metallic  Scales.  Assay  of  Ores  Containing 
Free  Gold.  The  Assay  of  Slags  and  Cupels. 

CHAPTER  XI 

ERRORS  IN  THE  ASSAY  FOR  GOLD  AND  SILVER         ,    .     .   120-134 

Losses  in  the  Cupellation  of  Pure  Silver.  Of  Pure  Gold.  How 
the  Losses  Occur.  Effect  of  Temperature.  Influence  of  Different 
Types  of  Cupels.  Curves  Showing  Cupellation  Losses.  Losses 
in  the  Cupellation  of  Gold-Silver  Alloys.  Relative  Amount  of 
Loss  by  Absorption  and  Volatilization.  Slag  Loss  and  Cupel  Ab- 
sorption in  Telluride  Ores.  In  Zinciferous  Material.  In  High- 
grade  Silver  Ores.  In  Cupriferous  Material.  General  Discussion 
of  Losses.  Other  Errors.  Retention  of  Lead  or  Copper  in  Beads. 
Retention  of  Silver  by  Gold  after  Parting.  Loss  of  Gold  by  Solution 
in  Acid.  Occluded  gases.  Error  in  Weighing.  Resume. 

CHAPTER   XII 

THE  ASSAY  OF  BULLION      .  135-146 

Classification  of  Bullions.  Assay  of  Lead  Bullion.  The  Assay  of 
Silver  Bullion.  Cupellation  Method,  Preliminary  Assay,  Check 
Assay,  Regular  Assay.  Gay-Lussac  Method  for  Silver  Bullion. 
Standardization  of  Solution,  Apparatus  Required,  the  Assay,  Cal- 


x  CONTENTS 

PAGES 

culations.  The  Assay  of  Gold  Bullion.  Preliminary  Assay,  Check 
Assay,  Proof  Alloys.  Regular  Assay.  Preparation  of  Proof  Gold 
and  Silver. 

CHAPTER   XIII 

THE  ASSAY  OF  ORES  AND  ALLOYS  CONTAINING  PLATINUM.  IRIDIUM, 

GOLD,  SILVER,  ETC 147-154 

Difficulty  of  Assay.  Composition  of  Platinum  Nuggets.  Cu- 
pellation  of  Lead  Containing  Platinum,  etc.  Appearance  of 
Cupeled  Bead.  Action  of  Acids  on  Metals  Contained  in  the  Plati- 
num-Silver bead.  Nitric  Acid.  Sulphuric  Acid,  Nitro-hydrochloric 
Acid.  Methods  of  Assay  to  Obtain  Lead  Button.  For  Ores  Con- 
taining Metallic  Grains.  For  Alloys.  Method  of  Assay  by  Part- 
ing Silver-Platinum  Alloys  in  Sulphuric  Acid,  etc.  Method  of 
Assay  by  Dissolving  Lead  Button  in  Nitric  Acid.  Results  Ob- 
tainable. 

CHAPTER   XIV 

THE  ASSAY  OF  TIN,  MERCURY,  LEAD,  BISMUTH  AND  ANTIMONY      .     .   155-164 

General  Remarks  on  the  Fire  Assay  for  Base  Metals.  The  Assay 
of  Tin  Ores.  Causes  of  Loss  in  the  Assay.  Preparation  of  the  Ore 
for  Assay.  The  Cyanide  Method.  The  German  Method.  Results 
Obtainable.  The  Mercury  Assay.  Apparatus  Required.  The 
Conduct  of  the  Assay.  Results  Obtainable.  Assay  of  Lead  Ores. 
Inaccuracies  of  it.  Lead  Flux  Method.  Soda-Argol  Method. 
Cyanide  Method.  Reactions  in  the  Assay.  Results  Obtainable. 
The  Assay  of  Antimony  and  Bismuth  Ores. 

INDEX 167-178 


LIST    OF    ILLUSTRATIONS 


FIG.  PAGE 

1.  Two-Muffle  Furnace.     Perspective  View 2 

2.  Two-Muffle  Furnace.     Cross-Section 3 

3.  Two-Muffle  Furnace.      Longitudinal  Section 4 

4.  Three-Muffle  Furnace.      Cross-Section         5 

5.  Three-Muffle  Furnace.     Longitudinal  Section 6 

6.  Wood -Burning  Muffle-Furnace.     Cross-Section 6 

7.  Wood-Burning  Muffle-Furnace.     Longitudinal  Section 7 

8.  Muffle-Furnace  for  Burning  Coke 7 

9.  Gasolene  Furnace  Apparatus 8 

10.  The  Carey  Gasolene  Burner 8 

11.  Gasolene  Tank  and  Pump  Apparatus 9 

12.  Gasolene-Burning  Crucible  Furnace 10 

13.  Gasolene-Burning  Muffle-Furnace n 

14.  Gas-Burning  Muffle-Furnace 12 

15.  Crucible  Tongs.     Undesirable  Model 13 

16.  Crucible  Tongs 13 

17.  Cupel  Tongs 13 

18.  Scorifier  Tongs 13 

19.  Scorifier  Tongs 13 

20.  Multiple  Scorifier  Tongs  (Keller) 14 

21.  Multiple  Scorifier  Tongs  (Keller) 14 

22.  Cupel  Charging  Device  (Keller) 15 

23.  Crucible  Pouring  Molds          16 

24.  Cupel  Tray 16 

25.  Fire-Clay  Annealing  Cup-Tray 16 

26.  Fire-Clay  Crucibles 17 

27.  Scorifiers 18 

28.  Buck  Board  and  Muller 18 

29.  Buck  Board  Brushes 19 

30.  Jones  Riffle  Sampler 32 

31.  Umpire  Ore  Sampler 33 

32.  Diagram  of  Assay  Balance 35 

33.  Pulp  Balance 39 

34.  Assay  Button  Balance '  •  4° 

35.  Platinum  Assay  W'eights 42 

36.  Assay-Ton  Weights 42 

37.  Gram  Weights 42 


xii  LIST   OF    ILLUSTRATIONS 

FIG.  PAGE 

370.  Freezing-Point  Curve.     Rhodonite  Hypersthene 58 

38.  Cupel  Machine 69 

39.  Freezing  Point  Curve  of  Lead -Silver 74 

40.  Freezing  Point  Curve  of  Lead-Copper 74 

41.  Parting  Bath 83 

42.  Parting  Flasks         83 

Curves  Showing  Silver  Losses  in  Cupellation 124 

43.  Jeweler's  Rolls 143 

44.  Apparatus  Required  for  the  Mercury  Assay 161 


A   MANUAL    OF    FIRE    ASSAYING 


ASSAY   FURNACES  AND  TOOLS 

THE  furnaces  used  in  assaying  are  many  in  design,  varying 
mainly  with  the  kind  of  fuel  used.  The  furnaces  are  classified 
as  follows:  (i)  Pot  furnaces,  in  which  the  assay  is  in  direct  contact 
with  the  fuel;  (2)  Muffle-furnaces,  in  which  a  muffle  or  receptacle 
containing  the  assay  is  externally  heated. 

As  the  muffle-furnace  is  practically  essential *  for  carrying  on 
the  operations  of  scorification  and  cupellation,  and  crucible  fusions 
can  be  made  satisfactorily  in  the  muffle  if  it  be  large  enough, 
muffle-furnaces  have  largely  replaced  pot  furnaces  for  general 
assaying.  In  general,  they  are  cleaner,  more  easily  operated, 
better  controlled  as  to  temperature,  and  if  large  enough  are  of 
great  capacity,  which  makes  them  especially  desirable  for  smelter, 
mill  and  mine  assay  offices,  where  frequently  a  great  number  of 
assays  are  performed  daily.  The  choice  of  fuel  for  heating  the 
furnaces  is  usually  dependent  on  locality.  Bituminous  and  lignite 
coal,  coke,  anthracite,  gasolene,  wood,  fuel  and  illuminating  gas 
are  all  used.  Of  these,  coke  and  anthracite  are  the  fuels  least 
desirable  for  muffle-furnaces,  as  these  fuels,  being  flameless,  must 
surround  the  muffle.  This  makes  the  firing  difficult,  requiring 
considerable  attention.  The  best  fuel,  usually  also  the  most 
easily  obtainable,  is  bituminous  or  good  lignite  coal,  yielding  a 
long  or  reasonably  long  flame.  One-,  two-  and  three-muffle 
furnaces,  constructed  of  fire-clay  tiling,  fire-brick,  and  common 
hard  brick,  tightly  bound  with  stays  and  rods,  are  in  common 
use,  and  for  general  utility,  where  much  work  must  be  turned 
out,  are  very  desirable. 

Fig.  i  shows  such  a  two-muffle  furnace  in  perspective,  and 
Figs.  2  and  3,  in  cross-section.  The  essential  parts  of  the  furnace, 

1  "Koenig's  Furnace,"  in  Trans.  A.  I.  M.  E.,  XXVIII,  p.  271.  This  furnace 
is  practically  a  pot  furnace  fired  by  gasolene,  and  with  an  air  blast  can  be  used  to 
scorify  and  cupel  without  a  muffle. 


A   MANUAL  OF   FIRE   ASSAYING 


as  the  tiling,  A,  B,  L,  K,  etc.,  can  be  readily  purchased,  although 
the  interior  of  the  furnace  may  also  be  built  of  fire-brick.  The 
tiling  furnace,  however,  is  more  easily  set  up  and  is  more  durable. 
In  the  design  of  the  soft-coal  furnace,  the  essential  dimensions 
are:  area  of  fire-grate;  distance  from  the  grate  to  the  bottom  of 
the  lower  muffle;  the  "fire  space,"  i.e.,  the  distance  between 
muffles  and  side  and  end  of  furnace,  and  between  the  top  of  the 
upper  muffle  and  the  top  of  the  furnace,  giving  the  proper  space 
for  combustion  of  the  gases.  These  dimensions  depend  upon  the 
nature  of  the  coal.  In  Figs.  2  and  3  the  grate  area  is  17.25  x 


FIG.  i.  —  TWO-MUFFLE  FUR- 
NACE.   Perspective  view 

21.0  in.;  distance  from  grate  to  lower  muffle,  18  in.;  fire  space, 
2.5  in.;  external  dimension  of  muffle,  19  in.  long,  12.25  m-  wide, 
7.75  in.  high.  The  flue  should  be  from  one-sixth  to  one-eighth  of 
the  grate  area.  It  is  best  placed  forward  of  the  center  of  the 
muffles  to  get  the  full  sweep  of  the  flame  around  them,  although 
this,  with  poor  draft,  is  apt  to  cause  smoky  muffles. 

The  walls  of  the  furnace  are  thick  (13  in.)  to  prevent  radiation. 
The  front  of  the  furnace  above  the  muffle  is  arched.  The  arch 
tiling  has  in  it  a  duct,  leading  to  the  flue,  to  carry  off  lead  fumes. 
The  muffles  are  supported  by  two  sets  of  tiles,  set  into  the  side 


ASSAY    FURNACES  AND  TOOLS  3 

walls  or  into  the  rear  end  wall.  These  tiles  frequently  prove 
weak,  and  in  falling  away  leave  the  muffle  without  support, 
causing  it  to  be  short  lived.  The  supports  are  best  made  in 
such  shape,  of  two  pieces,  that  they  will  join  under  the  center 
line  of  the  muffle  and  arch  over,  supporting  each  other.  The 


FIG.  2.  —  TWO-MUFFLE  FURNACE.     Cross-section 

writer  has  used  supports  of  this  type,  which  were  perfectly  satis- 
factory and  increased  the  life  of  the  muffles  greatly.  A  furnace 
of  the  kind  described  has  a  capacity  of  25  to  30  fusions  (2O-gram 
crucible)  per  hour,  including  the  necessary  cupellations.  If  the 
fusions  are  made  in  3o-gram  crucibles  or  in  2.5-in.  scorifiers,  the 


4  A   MANUAL  OF   FIRE  ASSAYING 

capacity  is  from  20  to  24.  With  good  draft,  this  furnace  burns 
from  37  to  47  Ibs.  of  coal  per  hour,  which,  at  $7.00  per  ton,  makes 
the  cost  per  assay  for  fuel  amount  to  from  O.65C.  to  o.8oc.  With 
a  good  grade  of  coal  (6500  to  7500  calories),  a  maximum  temper- 
ature of  1 150°  to  1200°  C.  can  be  obtained  in  this  furnace.  Figs. 
4  and  5  show  a  three-muffle  furnace  of  similar  type. 


FIG.  3. —  TWO-MUFFLE  FURNACE.     Longitudinal  section 

Coal  furnaces  may  also  be  readily  modified  to  burn  crude  oil. 
This  can  be  done  by  placing  tiling  in  the  fire-box,  and  making 
the  necessary  pipe  and  burner  connections.1  Figs.  6  and  7  show 
a  wood-burning  muffle-furnace.2  In  some  districts  wood  is  the 
only  available  cheap  fuel.  If  the  fire-box  and  fire  spaces  are 
properly  designed  (i.e.,  of  larger  size  than  in  the  coal  furnace) 
and  a  deep  bed  of  fuel  is  provided  for  (i.e.,  the  distance  from  the 

1  F.  C.  Bowman,  "Crude  Oil  in  Fire  Assaying,"  in  "Eng.  and  Min.  Journ.," 
LXXIX,  p.  221. 

2  See  also  E.  H.  Nutter,  in  "  Min.  and  Sci.  Press,"  XCII,  p.  329;  Louis  Janin, 
Jr.,  in  "Eng.  and  Min.  Journ.,"  LXXIV,  p.  810. 


ASSAY   FURNACES  AND  TOOLS  5 

grate  surface  to  the  bottom  of  the  fire-door  is  from  8  to  10  in.), 
sufficient  temperature  for  ordinary  assaying  can  be  attained  in 
this  type  of  furnace.  Almost  any  wood  may  be  used. 

Coke  and  anthracite  muffle-furnaces,  when  used,  are  usually 
smaller,  although  large  furnaces  may  be  specially  designed  and 
built  of  the  general  type  of  the  coal  furnaces  described. 

Fig.  8  shows  a  small  coke  or  anthracite  furnace.  The  fuel 
is  fed  in  at  the  top  and  kept  well  heaped  around  the  muffle.  A 
furnace  of  the  kind  shown  in  Fig.  8  will  consume  from  32  to  38 


FIG.  4.  —  THREE-MUFFLE  FURNACE.     Cross-section 

Ibs.  of  coke  per  hour,  according  to  draft.  With  a  muffle  1 1  x  16 
x  7  in.,  10  assays  per  hour,  including  cupellation,  can  be  made 
at  a  cost  of  i.yc.  per  assay  with  coke  at  $10  per  ton. 

Fire-clay  muffles  for  furnaces  are  made  in  varying  sizes  and 
shapes.  The  best  shape  for  general  use  is  one  of  nearly  rect- 
angular cross-section,  with  but  a  slightly  arched  top.  The  larg- 
est muffles  ordinarily  used  are  19  in.  long,  14.5  in.  wide  and 
7.75  in.  high  (outside  dimensions).  Muffles  19  in.  long,  12  in. 


6  A   MANUAL  OF   FIRE  ASSAYING 

wide  and  7.75  in.  high  are  very  common  in  coal  furnaces.  The 
muffles  have  two  holes  in  the  rear  end  to  induce  an  air  draft 
through  them. 

Gasolene-fired  Furnaces.  —  Furnaces  of  this  type  are  in  com- 
mon use,  and  for  small  offices,  where  the  pressure  of  work  is  not 
great,  they  afford  a  convenient  and  cheap  method  of  operation. 
Gasolene,  on  account  of  ease  of  transportation  and  great  calorific 
power,  is  also  employed  in  out-of-the-way  districts  for  extensive 


FIG.  5.  —  THREE-MUFFLE  FURNACE. 
Longitudinal  section 


FIG.  6.  —  WOOD-BURNING  MUF- 
FLE-FURNACE.     Cross-section 


daily  work.  Where  coal  is  reasonably  cheap,  not  above  $6.50 
per  ton,  gasolene  at  3oc.  per  gal.  cannot  compete  with  it  in  large 
offices  or  schools,  where  the  assay  furnaces  are  operated  con- 
tinuously for  the  greater  part  of  the  day. 

Fig.  9  shows  a  gasolene  furnace  apparatus.  The  furnace, 
divided  into  crucible  and  muffle  compartments,  is  made  of  fire- 
clay tiling,  bound  with  sheet  iron.  It  is  heated  by  a  brass  and 
copper  burner,  provided  with  a  generating  device.  The  burners 


ASSAY    FURNACES  AND  TOOLS 


FlG.    7.  —  WOOD-BURNING  MUFFLE-FURNACE. 

Longitudinal  section 


FIG.  8.  — MUFFLE-FURNACE  FOR 
BURNING  COKE 


8 


A  MANUAL  OF   FIRE  ASSAYING 


are  made  in  varying  sizes  to  suit  different  furnaces.  The  gaso- 
lene is  stored  in  a  steel  tank,  of  5  or  10  gal.  capacity,  provided 
with  an  air  pump  to  furnish  pressure.  A  pressure  gage  is 


FIG.  9.  —  GASOLENE  FURNACE  APPARATUS 

attached  to  the  tank.  Generally,  0.25-  to  o.375-in.  piping  joins 
the  tank  and  the  burner.  The  burner  and  piping  are  connected 
by  a  special  universal  joint,  so  that  the  burner  can  be  swung  into 
and  out  of  position.  The  burner  (if  the  Carey)  should  fit  tightly 
against  the  fire-clay  ring  or  boss  in  the  opening  of  the  furnace,  so 
that  all  the  air  for  the  combustion  of  the  gasolene  is  drawn  in 
through  the  burner  tube.  To  insure  tight  joints,  glue  or  soap, 
not  white  or  red  lead,  must  be  used  in  the  screw  connections. 


FIG.  10.  —  THE  CAREY  GASOLENE  BURNER 

The  gasolene  is  fed  to  the  burner  under  a  pressure  of  10  to  20 
Ibs.,  though  for  special  purposes  higher  pressures  are  used. 

Fig.    10  shows  a  detailed  view  of  the  Carey  burner.     The 


ASSAY    FURNACES  AND  TOOLS  9 

upper  valve  controls  the  main  gasolene  supply,  and  the  lower 
one  controls  the  generator.  The  burner  is  heated  by  the  gener- 
ator, so  that  the  gasolene  issuing  from  the  main  needle-valve  is 
vaporized,  and  in  its  passage  to  the  furnace  draws  in  air  through 
the  burner  tube,  the  mixture  igniting  and  burning  at  the  mouth 
of  the  burner  in  the  hot  furnace.  Burners  are  listed  by  the  diam- 
eter of  their  tubes.  Five  sizes  are  made,  from  1.25  to  2.25  in., 
each  size  varying  by  0.25  in. 


FIG.  ii.  —  GASOLENE  TANK  AND  PUMP 
APPARATUS 

Fig.  1 1  shows  the  tank  and  pump  apparatus.  It  is  best  to 
place  this  at  a  considerable  distance  from  the  furnace,  in  order 
to  avoid  accidental  explosions.  Fig.  12  shows  a  crucible  furnace, 
and  Fig.  13  a  large  gasolene  muffle-furnace.  The  writer  has 
attained  a  temperature  of  1350°  C.  in  small  gasolene  furnaces, 
such  as  Fig.  12  represents,  and  1250°  C.  in  large  furnaces,  as 
represented  by  Fig.  13.  By  a  special  construction  of  furnace, 


io  A   MANUAL  OF   FIRE   ASSAYING 

with  graphite  muffle  and  heavy  insulation  against  radiation,  with 
good  draft,  the  writer  has  attained  (for  metallurgical  experimen- 
tation) temperatures  of  1500°  to  1530°  C,  after  three  hours,  by 
a  2-in.  gasolene  burner  as  shown  in  Fig.  io,  with  gasolene  at  a 
pressure  of  55  Ibs.  and  a  consumption  of  1.53  gal.  per  hour.  A 
2-in.  Carey  burner,  under  io  Ibs.  pressure,  will  consume  from 
0.65  to  0.75  gal.  per  hour.  A  No.  31  Carey  combination  furnace, 
holding  at  a  charge  in  the  crucible  compartment  six  2O-gram 
crucibles  and  having  a  muffle  yx  10.5x4.5  in.  in  size,  has  a 
capacity  of  io  fusions  per  hour,  including  cupellation.  With 
gasolene  at  3oc.  per  gallon,  the  cost  of  fuel  per  assay  is  2.25C. 


FIG.  12.  —  GASOLENE-BURNING 
CRUCIBLE  FURNACE 

Gas  Furnaces.  —  Where  municipal  illuminating  gas  or  other 
gaseous  fuel  is  available,  gas-fired  furnaces  are  convenient  and 
cheap  of  operation.  The  Reichhelm  furnace  (American  Gas 
Furnace  Company)  is  frequently  used.  The  furnaces  require  air  at 
low  pressure,  which  is  mixed  with  gas  in  proper  proportion  before 
it  enters  the  furnace  through  the  several  burners.  The  propor- 
tion of  gas  to  air  is  controlled  by  valves.  Fig.  14  shows  the 
furnace.  Gas  furnaces  permit  of  close  control  of  heat  and  are 
desirable  for  accurate  temperature  work. 


ASSAY    FURNACES  AND  TOOLS 


1 1 


Furnace  Tools.  —  Convenient  tools  are  necessary  for  the  hand- 
ling of  crucibles,  scorifiers  and  cupels.  The  features  essential 
in  these  tools  are  that  they  be  light,  grasp  the  crucible,  etc., 
firmly,  with  no  danger  of  tipping,  and  take  up  little  room  in  the 
furnace.  As  an  illustration  of  a  tool  deficient  in  these  qualities 


FIG.  13.  —  GASOLENE-BURNING  MUFFLE-FURNACE 

and  therefore  undesirable,  Fig.  15  is  given.  This  shows  a  pair 
of  crucible  tongs  designed  to  grasp  the  body  of  the  crucible.  It 
cannot  be  handled  in  a  muffle  full  of  crucibles,  owing  to  the  space 
it  takes  up  in  opening.  Fig.  16  shows  a  pair  of  crucible  tongs  to 
grasp  the  sides  of  the  crucible,  and  operating  in  little  space. 


12 


A  MANUAL  OF   FIRE  ASSAYING 


Fig.  17  shows  two  types  of  cupel  tongs.     Fig.  18  shows  a  good 
form  of  scorifier  tongs,  and  Fig.  19  another  form. 

For  large  offices  where  much  work  must  be  quickly  accom- 
plished, special  forms  of  tools  may  be  used.  Figs.  20  and  21 
show  a  multiple  tongs  l  for  scorifiers.  This  apparatus  will  handle 


FlG.    14.  —  GAS-rBURNING    MUFFLE-FURNACE 

20  scorifiers,  practically  a  muffleful  at  one  time.  It  is  composed 
of  quintuple  tongs,  corresponding  to  the  five  longitudinal  rows 
of  scorifiers  in  the  muffle.  The  lower  part  of  each  pair  of  the 
tongs  consists  of  a  fork  on  which  the  scorifiers  rest,  and  one  of 
whose  prongs  is  rectilinearly  extended  through  two  bearings  in  a 

1  Edward  Keller,  "Labor-Saving  Appliances  in  the  Works  Laboratory,"   in 
Trans.  A.  I.  M  E.,  XXXVI,  p.  3. 


ASSAY    FURNACES  AND  TOOLS  13 

frame  and  held  in  position  by  collars.  This  extension  is  free  to 
revolve  on  the  bearings,  and  it  is  the  axis  of  rotation  of  the  tongs. 
To  each  of  them  is  attached,  at  a  right  angle,  a  lever  extending 
upward  at  45°,  and  all  the  levers  are  connected  by  slotted  joints 
to  a  cross-rod.  Therefore  if,  by  means  of  a  crank  fastened  to 


FIG.  is-  —  CRUCIBLE  TONGS.     Undesirable  model 


FIG.  1 6.— CRUCIBLE  TONGS 


FIG.  17.  — CUPEL  TONGS 


FIG.  18.  —  SCORIFIER  TONGS 


FIG.  19.  — SCORIFIES  TONGS 

the  end  of  one  of  the  extended  prongs,  one  of  the  forks  is  turned 
and  the  scorifiers  tilted  to  the  desired  angle,  the  others  rotate  to 
the  same  extent.  The  center  of  gravity  of  the  scorifiers  lies  to 
one  side  of  the  rotation  point,  and  they  would  therefore,  on  being 
lifted,  tilt  in  that  direction;  this,  however,  is  prevented  by  the 
cross-bar  resting  against  a  post  at  that  end  of  the  frame  toward 


14  A  MANUAL  OF   FIRE  ASSAYING 

which  the  inclination  tends.  The  scorifiers  are  clutched  by  the 
upper  prongs  of  the  tongs,  which  is  fastened  to  a  spring  on  a  post 
of  the  fork  below,  and  which  is  free  to  move  in  a  vertical  plane, 


FIG.  20. —  MULTIPLE  SCORIFIER  TONGS.     (Keller) 


FIG.  21. —  MULTIPLE  SCORIFIER  TONGS.     (Keller) 

the  pivotal  point  lying  over  the  spring  and  post.  By  bringing 
pressure  on  the  extended  ends  of  these  clutch  bars  behind  the 
pivot,  their  other  end  will  rise  above  the  scorifiers,  and  thus 
release  them,  or  permit  the  placing  of  them  onto  the  tongs.  The 


ASSAY    FURNACES  AND  TOOLS  15 

pressure  exerted  on  the  rear  ends  of  the  clutches  is  accomplished 
by  means  of  a  cross-bar  fastened  to  a  spring  bar,  which  is  itself 
fastened  to  the  handle  of  the  instrument.  An  ordinary  mold 
with  20  holes,  arranged  to  receive  the  contents  of  the  scorifiers, 
goes  with  the  tongs. 

Fig.  22  shows  a  device  to  charge  30  cupels  at  one  time.  It 
comprises  a  top  sliding  plate  with  openings  corresponding  exactly 
to  the  position  of  the  cupels.  The  openings  in  the  lower  plate 
correspond  exactly  with  those  of  the  upper  one;  the  plate,  how- 
ever, rests  on  two  adjacent  sides  extended  downward  at  right 
angles  to  the  plate  and  to  each  other,  thus  forming  two  closed 
sides  of  the  instrument;  one  at  the  front  and  the  other  at  the  right- 
hand  side.  The  hight  of  these  sides  is  such  that  when  resting 
on  the  bottom  of  the  muffle  the  bottom  plate  will  be  some  dis- 


FIG.  22. —  CUPEL  CHARGING  DEVICE.     (Keller) 

tance  above  the  cupels,  and  by  a  slight  pull  forward  and  a  push 
to  the  left  with  the  handle  of  the  instrument  the  set  of  cupels 
will  be  perfectly  alined  in  both  directions  and  the  apertures  in 
the  lower  plate  will  exactly  cover  the  tops  of  the  cupels.  The 
lead  buttons  are  placed  in  the  apertures  of  the  upper  plate  and 
rest  on  the  lower  plate  before  introducing  the  instrument  into  the 
furnace,  and  when  it  is  placed  over  the  cupels,  which  have  been 
properly  alined  in  the  muffle,  the  upper  plate  is  pushed  forward 
to  a  stop-point,  bringing  the  apertures  of  the  two  plates  into 
register,  thus  causing  the  lead  buttons  to  drop  down  into  the 
cupels.  The  handle  of  the  upper  plate  runs  through  guides  fixed 
to  the  handle  of  the  lower  plate;  both  handles  are  connected 
with  a  spring,  which  acts  as  a  brake  when  the  upper  plate  is 
pushed  forward  to  drop  the  buttons,  and  also  serves  to  bring  it 


i6 


A   MANUAL  OF   FIRE  ASSAYING 


back  into  its  original  position,  in  which  the  buttons  cannot  drop 
through  the  apertures  in  the  lower  plate. 

Molds.  —  Fig.  23  shows  machined  cast-iron  molds  to  receive 


the  molten  fusions.  The  sharp  cone-shaped  mold  is  preferable 
to  the  shallow  hemispherical  type,  as  the  lead  buttons  are  then 
sharp  and  well  defined  and  separate  easily  from  the  slag.  The 
mold  is  best  made  with  a  screw-handle,  so  as  to  be  easily  repaired 
in  case  of  breakage.  The  inner  surface  of  the  molds  should  be 


FIG.  24. —  CUPEL  TRAY 

machined  smooth,  to  permit  the  ready  separation  of  slag  and 
lead  button  from  the  mold. 

For  scorification  fusions,  smaller  molds  are  often  used.  For 
the  transfer  of  cupels  to  the  parting  room,  iron  cupel  trays,  as 
illustrated  in  Fig.  24,  are  used.  The  handle  is  removable,  and 
one  handle  serves  for  a  number  of  trays.  For  the  annealing  of 
gold  beads,  or  cornets,  fire-clay  trays  as  shown  in  Fig.  25  are 


FIG.  25.  —  FIRE-CLAY  ANNEALING  CUP-TRAY 


ASSAY    FURNACES  AND  TOOLS 


employed.  Fire-clay,  however,  is  very  easily  broken,  and  more 
satisfactory  trays  are  made  of  sheet  iron  and  heavy  asbestos 
board. 

Crucibles  and  Scarifiers.  —  Fire-clay  crucibles  are  largely  used 
in  the  United  States,  and  fire-clay  ware  for  assay  purposes  is 


FIG.  26.  —  FIRE-CLAY  CRUCIBLES 

made  to  a  large  extent  in  some  of  the  western  States.     Following 
is  the  analysis  of  a  Colorado  crucible  clay: 

Loss  on  ignition 10.14  per  cent. 

Alumina I5-°9  Per  cent. 

Silica    71.81  per  cent. 

Ferric  oxide 1.75  per  cent. 

Lime 0.14  per  cent. 

Magnesia 0.05  per  cent. 

Alkalies 1.02  per  cent. 

The  crucibles  are  rated  by  "gram"  capacity,  that  is,  by  the 
number  of  grams  of  ore  with  the  proper  amount  of  fluxes  neces- 
sary for  fusion  which  the  crucible  will  hold.  The  chief  sizes  are 
5,  10,  12,  15,  20,  30  and  40  grams;  of  these  the  20-  and  3o-gram 
sizes  are  mostly  used,  the  2O-gram  crucible  for  the  0.5  assay  ton, 
and  the  3o-gram  for  the  i  assay  ton  fusions.  Fig.  26  shows  the 


i8  A   MANUAL  OF   FIRE   ASSAYING 

various  shapes  employed.  Imported  Hessian  triangular  crucibles 
and  sand  crucibles  are  also  used,  but  in  small  quantities.  Scori- 
fiers  are  made  of  the  same  clays  as  the  crucibles  and  are  designated 


FlG.    27. SCORIFIERS 

in  size  by  their  outside  diameters;  i.  5-,  2-, 2. 5-  and  3. 5-inch  sizes 
are  made.  These  will  hold  a  volume  of  15  c.c.,  25  c.c.,  37  c.c. 
and  100  c.c.,  respectively.  The  2.5-inch  scorifier  is  the  one  com- 
monly used.  Fig.  27  shows  the  ordinary  type  of  scorifiers. 
Roasting  dishes  are  shallow  fire-clay  dishes  similar  to  scorifiers, 
but  not  so  thick.  They  are  rated  by  their  diameters;  the  common 
sizes  being  3,  4,  5  and  6  inches.  Fig.  28  shows  the  ordinary 


FIG.  28.  —  BUCK  BOARD  AND  MULLER 


buck  board  and  muller,  and  Fig.  29  buck  board  brushes.  For 
the  description  of  other  minor  tools  and  apparatus,  as  screens, 
pliers,  and  crushing  and  grinding  machinery,  necessary  to  the 
assay  laboratory,  the  reader  is  referred  to  the  voluminous  and 


ASSAY    FURNACES   AND   TOOLS 


well-illustrated  catalogues  of  the  assay  supply  houses.  Balances, 
weights,  sampling  tools,  cupels,  parting  devices,  etc.,  are  dis- 
cussed under  their  respective  chapters. 


FIG.  29. —  BUCK  BOARD  BRUSHES 


II 


DEFINITIONS;  REAGENTS;  THE  ASSAY  OF 
REAGENTS 

ASSAYING  includes  all  those  operations  of  analytical  chemistry 
which  have  for  their  object  the  determination  of  the  constituents 
of  ores  and  metallurgic  products.  Three  methods  are  used: 
(i)  Fire  assaying  (dry  methods);  (2)  gravimetric  analysis  (wet 
methods);  (3)  volumetric  and  calorimetric  analysis  (wet  methods). 
This  work  treats  of  fire  assaying  only,  with  a  few  exceptions. 
The  quantitative  determination  of  the  following  metals  is  dis- 
cussed: gold,  silver,  platinum,  etc.,  lead,  antimony,  bismuth,  tin 
and  mercury;  chiefly,  however,  gold  and  silver. 

Fire  assaying  comprises  the  separation  of  the  metal  sought 
from  the  other  components  of  the  ore,  by  heat  and  suitable 
fluxes,  and  then  the  weighing  of  it  in  a  state  of  greater  or  lesser 
purity. 

Gold  and  Silver.  —  Gold  and  silver  are  determined  in  their 
ores,  or  metallurgic  products,  by  collecting  them  with  lead, 
forming  an  alloy,  which  may  be  accomplished  either  by  the 
crucible  or  the  scorification  fusion,  the  lead  being  then  driven  off 
by  cupellation,  and  the  resultant  bead  of  the  gold  and  silver 
alloy  weighed.  The  separation  of  gold  from  silver  is  accom- 
plished by  parting  in  most  instances  with  nitric  acid,  rarely  by 
sulphuric  acid. 

In  order  to  successfully  collect  the  precious  metals  by  means 
of  lead,  it  is  essential  that  the  ore  be  mixed  with  suitable  fluxes, 
so  that  in  fusion  the  ore  is  thoroughly  decomposed  chemically, 
and  a  liquid  slag  of  the  proper  constitution  produced,  enabling 
the  lead  with  its  alloyed  gold  and  silver  to  settle  from  the  slag 
by  gravity,  thus  affording  a  ready  separation. 


DEFINITIONS-REAGENTS;   THE   ASSAY  OF   REAGENTS     21 
REAGENTS   COMMONLY  USED   IN  ASSAYING 


NAME 

FORMULA 

NATURE  (CHEMICAL) 

i     Litharge           

PbO 

basic 

2     Sodium  carbonate    

Na2CO3 

basic 

3.   Sodium  bicarbonate  
4    Potassium  carbonate 

NaHCO3 
K2CO3 

basic 
basic 

5    Silica 

SiO* 

acid 

6.  Borax                 

Na2B4O7.ioH,O 

acid 

7.   Borax  glass                        

Na2B4O7 

acid 

8    Fluorspar  1 

CaF2 

neutral 

9.   Lime        

CaO 

basic 

10.  Hematite  
ii.   Test  lead   ") 

Fe203 
Pb 

basic 
basic 

Sheet  lead  ) 
1  2    Argol 

KHC4H4OG 

basic 

j  2     Charcoal      

c 

14    Coke  dust                                   •      •  • 

1  5  .   Flour 

1  6.  Lead  flux                   

17.  Black  flux 

1  8.   Black  flux  substitute 

19.  Potassium  cyanide  

KCN 

neutral 

20.   Potassium  nitrate  
21.   Salt  (sodium  chloride)   

KN03 
NaCl 

basic 
neutral 

1.  Litharge  is  acted  on  in  the  crucible  by  reducing  agents, 
such  as  charcoal,  etc.,  and  metallic  lead  produced  as  follows: 

2PbO  +  C  =  2Pb  +  C02 

The  litharge  not  reduced  is  acted  on  by  silica  and  borax  glass, 
producing  silicates  and  borates  of  lead,  as  follows: 

PbO  +  SiO2  =  PbSiO3,  etc. 

Litharge  melts  at  906°  C. 

2,  3.    Sodium  carbonate  and  bicarbonate  is  decomposed  by 
heat  in  the  crucible,  as  follows,  at  high  temperature: 

2NaHC03  =  Na20  +  H2O  +  2CO2 
Or,  in  the  presence  of  silica, 

Na2CO3  +  SiO2  =  Na2SiO3  +  CO2 

1  Not  decomposed  in  the  crucible  by  temperatures  ordinarily  used  in  assaying. 


22  A   MANUAL  OF   FIRE   ASSAYING 

The  Na2O,  with  silica,  forms  sodium  silicates,  as  Na2SiO3,  etc., 
which  are  very  fusible.  It  also  possesses  the  property  of  readily 
forming  sulphides  and  sulphates  and,  in  the  presence  of  metallic 
Fe,  of  freeing  lead  in  the  charge  from  sulphur. 

Na2CO3  melts  at  814°  C. 

4.  Potassium  carbonate  acts  in  a  similar  manner  to  sodium 
carbonate.     It  melts  at  885°  C. 

5.  Silica  is  a  powerful  acid  flux  and  combines  with  the  metallic 
oxides  or  bases  present  in  the  charge  to  form  the  slag,  which  is 
mainly   composed   of  silicates.     It   is   present   in    most   ores   in 
considerable  quantity,  ranging  from  small  amounts  in  basic  ores 
to  the  main  bulk  of  the  ore  in  quartz  ores.     It  melts  at  1775°  C.1 
(Quartz).  —  (Roberts-Austin,  1899.) 

6.  7.    Borax  and  borax  glass  act  as  an  acid  flux.     The  formula 
may  be  written  Na2O,  2B2O3.     It  contains  an  excess  of  boric 
acid,  which  can   unite  with  metallic  oxides  to  form  a  borate 
slag.     Boric  oxide  and  boric  acid  yield  a   very  fluid   slag  with 
zinc  oxide,  either  alone  or  with  one-half  its  weight  of  borax. 
It    acts    very   corrosively   on    clay   crucibles.     Borax    melts    at 
560°  C. 

8.  Fluorspar  is  occasionally  used  in  assaying.     It  fuses  at  a 
comparatively  high  temperature,  but  when  fused  is  very  thinly 
fluid.     The  greater  part  of  it  remains  unchanged  throughout  the 
fusion,  and  hence  its  lime  cannot  be  considered  as  available  for 
fluxing  silica.     It  gives  the  slags  containing  it  a  stony  appearance. 
Owing  to  its  great  fluidity,  it  has  the  property,  shared  by  soda 
and  litharge  to  some  extent,  of  holding  in  suspension  unfused 
particles,  thus  still  making  a  fluid  slag.     Where  the  decomposi- 
tion of  the  ore  to  be  assayed  is  essential,  as  it  is  in  most  cases, 
its  use  is  not  to  be  advocated. 

9.  Lime  is  used  either  as  the  carbonate  or  as  the  oxide  or 
hydrate.     In  the  crucible  it  is  converted  into  oxide,  the  carbonate 
beginning  to  lose  its  CO2  at  700°  C.     In  itself  it  is  extremely 
infusible  (1900°  C. ;  Hempel,  1903),  but  with  silica,  when  joined 
with  other  bases  and  in  moderate  quantities,  it  makes  very  de- 
sirable slags.     It  is  found  in  many  ores.     Magnesia  acts  in  a 
similar  way.     Its  melting-point  is  2250°  C.     (Hempel,  1903.) 

1  Day  and  Shepard  give  the  melting-point  of  SiO2  at  approximately  1625°  C.; 
"  Journ.  Am.  Chem.  Soc.,"  XXVIII,  p.  1096. 


DEFINITIONS;   REAGENTS;   THE    ASSAY   OF   REAGENTS      23 

10.  Hematite,  or  natural  ferric  oxide,  and  limonite,  are  of 
frequent  occurrence  in  ores,  and  are  sometimes  added  as  a  flux. 
Ferric  oxide  is  very  infusible.     In  the  crucible  it  is  converted  by 
reducing  agents,  such  as  argol,  charcoal,  etc.,  to  ferrous  oxide 
(FeO),  and  then  unites  with  silica  to  form  silicates.     The  fact 
that  it  is  reduced  to  ferrous  oxide,  conversely  gives  it  an  oxidizing 
power.     Manganese  oxides  acting  in  a  similar  way  are  also  fre- 
quently found  in  ores.     Alumina,  A12O3,  is  often  found  in  ores, 
and   unites  with  silica  to  form   silicates.     It   has  no  oxidizing 
power.     A12O3  melts  at  1880°  C.     (Hempel,  1903.) 

1 1 .  Test  lead  and  sheet  lead  are  used  chiefly  in  the  scorifica- 
tion  assay  and  in  cupellation.     In  both  of  these  operations  the 
lead  is  oxidized  by  the  oxygen  of  the  air  (2Pb  +  O2  =  2  PbO) 
to  litharge.     In  the  scorification  assay  part  of  this  PbO  volatil- 
izes; the  greater  part  becomes  fluid  and  holds  in  suspension  and 
solution  other  metallic  oxides  derived  from  ores,  thus  forming 
what  is  termed  an  oxide  slag.     In  cupellation,  part  of  the  lead 
is  volatilized  as  PbO,  and  part  is  absorbed  by  the  cupel  as  PbO. 
Lead  melts  at  326°  C. 

12.  Argol  is  a  crude  bitartrate  of  potassium,  separating  out 
in  wine  casks,  from  the  wine  on  standing.    On  heating,  it  breaks 
up  as  follows: 

2KHC4H406  +  heat  -  K2O  +  5H2O  +  6CO  +  2C 
The  carbon  and  carbon  monoxide  set  free  give  it  its  reducing 
power.     The  K2O  left  acts  as  a  basic  flux. 

13.  14,  15.    Charcoal,  coke,  coal    dust,  sugar   and   flour   are 
reducing   agents  by  virtue  of  the  carbon  or  hydrogen,  or  both, 
that  they  contain. 

1 6.  Lead  flux  is  a  ready-prepared  flux  used  mainly  in  the 
assay  of  lead  ores  for  lead.     It  has  the  following  composition: 

Sodium  bicarbonate 16  parts 

Potassium  carbonate 16  parts 

Borax  glass 8  parts 

Flour   4  parts 

It  is  also  made  up  in  other  proportions. 

17.  Black  flux  is  made  of  i   part   KNO3  and  3  parts  argol, 
deflagrated.     It  is  sometimes  used  in  the  tin  assay. 

1 8.  Black  flux  substitute  consists  of  3   parts  of  flour  and 
10  parts  of  NaHCO3.     It  is  used  in  the  tin  assay. 


24  A   MANUAL  OF   FIRE  ASSAYING 

19.  Potassium  cyanide  (KCN)  is  a  powerful  poison,  and  when 
powdering  it  great  care  must  be  taken  not  to  inhale  it.     The 
mortar  in  which  it  is  powdered  should  be  covered  by  a  cloth 
and  the  operation  conducted  at  an  open  window.     It  fuses  at 
526°  C.  (commercial,  98  per  cent.  KCN),  and  remains  unchanged 
at  a  fair  red  heat  when  air  is  excluded.     Crucibles  in  which  it  is 
used  for  fusion  should  be  covered.     It  is  a  powerful  reducing 
and  desulphurizing  agent,  acting  as  follows: 

PbO  +  KCN  =  KCNO  +  Pb 
PbS  +  KCN  =  KCNS  +  Pb 

It  is  used  mainly  in  the  assay  of  base  metals,  as  bismuth,  tin, 
etc.  The  cyanide  used  in  assaying  should  be  as  pure  as  possible; 
pure  sodium  cyanide  is  preferable  to  the  usually  more  or  less 
impure  potassium  cyanide.  No  material  rated  less  than  98  per 
cent.  KCN  should  be  used. 

20.  Potassium  nitrate  or  niter  is  used  as  an  oxidizing  agent. 
With  metallic  lead  it  acts  as  follows: 


yPb  +  6KNO3  =  yPbO  +  3K2O  +  3N2  +  4O2  (approximately). 

It  is  frequently  used  in  assaying  to  oxidize  impurities  in  the 
charge,  such  as  sulphur,  arsenic,  etc.  It  acts  as  a  basic  flux. 
Potassium  nitrate  fuses  at  339°  C. 

21.  Salt  (NaCl)  is  used  as  a  cover.  It  is  very  thinly  fluid 
and  is  not  decomposed  during  the  fusion.  It  melts  at  772°  C. 

The  Assay  of  Reagents.  —  It  is  essential  for  the  assayer  to  be 
assured  of  the  fact  that  his  reagents  are  pure,  or  at  least  to  know 
to  what  extent  they  are  impure  and  what  the  impurity  consists 
of.  For  this  reason  it  is  necessary  to  examine  lots  of  reagents 
from  time  to  time,  as  they  come  into  the  laboratory,  by  approved 
chemical  methods,  to  determine  their  purity.  Sometimes  re- 
agents or  fluxes,  as  a  result  of  being  left  exposed  in  the  laboratory, 
become  accidentally  or  purposely  "salted"  or  contaminated  with 
gold,  silver  or  base-metal  values.  A  blank  assay  for  metals  on 
the  reagents  will  readily  determine  this.  In  general,  it  may  be 
stated  that  the  labeling  of  a  chemical  "c.  p."  does  not  necessarily 
make  it  so. 

It  is  necessary  to  determine  the  silver  in  litharge  and  test 
lead,  as  these  two  reagents  frequently  contain  some  silver,  due 


DEFINITIONS;   REAGENTS;   THE   ASSAY   OF   REAGENTS      25 

to  their  being  usually  made  from  lead  bullion  refined  by  the 
Parkes'  or  ziric-desilverization  process,  which  leaves  some  silver 
in  them.  As  litharge  is  almost  invariably  used  in  the  crucible 
assay,  and  test  lead  in  the  scorification  assay,  any  silver  or, 
possibly,  gold  introduced  into  the  results  by  their  use  must  be 
subtracted,  so  as  not  to  be  ascribed  to  the  ores.  Most  assay 
supply  houses  now  furnish  practically  silver-free  litharge  and 
lead  containing  only  traces  of  silver  and  no  gold. 

The  method  of  determining  silver  and  gold  in  litharge  and 
test  lead  is  as  follows : 

The  following  charge  is  weighed  out  in  duplicate: 

Litharge 3  assay  tons 

Sodium  carbonate 20  grams 

Silica 7  grams 

Argol 2  grams 

Borax  glass 5  grams  (as  a  cover) 

The  various  ingredients  are  put  from  the  scale  pan  on  a  sheet 
of  glazed  paper  and  thoroughly  incorporated  by  mixing.  It  is 
essential  to  weigh  the  litharge  and  argol  as  accurately  as  possible 
with  the  pulp  balances  in  use. 

The  incorporated  charge  is  then  transferred  to  a  2o-gram 
crucible,  a  shallow  cover  of  borax  glass  being  put  on  top  of  the 
charge,  and  then  fused  in  the  muffle-furnace  for  from  25  to  35 
minutes  at  a  yellow  heat  (1000°  C).  The  fusion  is  considered 
complete  when  the  charge  is  in  quiet  fusion,  that  is,  when  there 
is  no  more  bubbling  and  boiling  in  the  charge  and  when  the 
only  motion  observable  is  that  due  to  convection  currents.  The 
charge  is  then  poured  into  an  iron  mold  and  allowed  to  solidify, 
which  takes  approximately  10  minutes.  The  lead  button  is  then 
separated  from  the  slag  by  the  hammer  and  formed  into  a  cube. 
It  is  weighed  and  its  weight  recorded  in  grams  and  tenths  of  a 
gram  in  the  assay  note-book,  a  definite  assay  number  being 
assigned  to  this  assay  and  its  duplicate.  The  lead  button  is  then 
cupeled,  the  cupel  being  first  placed  in  the  muffle  for  10  to  12 
minutes  before  the  lead  button  is  dropped  into  it.  If  the  button 
weighs  from  15  to  20  grams,  as  it  should,  it  will  take  25  or  30 
minutes  to  finish  the  cupellation,  that  is,  to  drive  off  the?  lead. 
The  end  of  this  operation,  in  this  particular  instance,  is  denoted 
by  the  darkening  of  the  small  silver  bead.  The  bead  is  then  re- 


26  A   MANUAL  OF   FIRE  ASSAYING 

moved  from  the  cupel  after  this  has  become  cold,  flattened  on 
a  small  anvil  with  a  blowpipe  hammer,  cleaned  of  adhering 
bone-ash  from  the  cupel  by  a  button  brush,  and  weighed  carefully 
on  the  assay  balances,  the  weight  being  recorded  in  milligrams 
and  hundredths  of  a  milligram.  The  weight  of  the  bead,  divided 
by  the  number  of  assay  tons  (3)  taken  in  the  assay,  gives  the 
number  of  ounces  contained  in  a  ton  (2000  Ibs.)  of  litharge,  or 
th£  number  of  milligrams  per  assay  ton  of  litharge.1  If  the 
presence  of  gold  is  suspected  in  the  litharge,  the  silver  bead  from 
the  cupellation,  after  weighing,  is  dropped  into  a  parting-cup 
filled  with  hot  nitric  acid  (9  parts  water  to  i  part  concentrated 
nitric  acid,  sp.  gr.  1.42),  which  will  dissolve  the  silver  and  leave 
the  gold  as  a  black  residue.  This  residue  is  washed  three  times 
by  decantation  with  cold  distilled  water,  carefully  dried  and 
annealed  at  a  red  heat  in  the  muffle;  after  cooling  it  is  weighed 
as  already  described  for  silver.  The  weight  of  the  gold  is  recorded 
and  then  subtracted  from  the  weight  of  the  original  gold  and  silver 
bead.  The  difference  in  weight  gives  the  amount  of  silver. 

To  determine  the  silver  and  gold  in  test  lead,  weigh  out  3 
assay  tons,  place  in  a  2.5-in.  scorifier,  add  a  pinch  of  borax  glass, 
and  scorify  in  the  muffle  at  a  yellow  heat  (1000°  C).  As  the 
lead  oxidizes  to  litharge,  this  melts  and  forms  a  slag  which, 
owing  to  the  convexity  of  the  meniscus  of  molten  lead,  falls  to 
the  side  of  the  surface  and  forms  the  slag  ring,  leaving  a  disk  of 
fresh  lead  exposed.  The  scorification  is  finished  when  the  slag 
finally  covers  all  the  lead.  The  charge  is  then  poured  into  an 
iron  mold,  the  further  method  of  procedure  followed  being  iden- 
tical with  the  one  described  for  the  litharge  assay. 

It  is  possible  to  obtain  test  and  sheet  lead  with  only  traces 
of  silver,  and  litharge  practically  free  from  silver.  It  is  often 
desirable  that  the  litharge  should  contain  a  uniform  amount  of 
silver,  for  whenever  low-grade  gold  ores,  deficient  in  silver,  are 
assayed,  silver  would  have  to  be  added  at  some  stage  of  the  assay 
in  order  to  assure  parting,  or  the  complete  separation  of  the 
gold  from  the  silver.  In  assaying  very  low-grade  gold  ores,  in 
which  practically  only  gold  is  present,  the  final  bead  might  be 
so  small  as  to  sink  into  minute  cracks  in  the  cupel  and  thus  be 

1  For  a  discussion  of  weights  used  in  assaying,  cupellation  and  weighing, 
reference  should  be  made  to  these  subjects. 


DEFINITIONS;   REAGENTS;  THE   ASSAY   OF   REAGENTS      27 

lost.  The  addition  of  silver  in  this  case,  either  by  adding  it  in 
the  metallic  state  or  by  its  presence  in  the  litharge,  obviates  this 
difficulty. 

Litharge  will  frequently  contain  from  0.20  to  0.32  mg. 
of  silver  per  assay  ton.  It  is,  however,  not  safe  to  assume  the 
above  figures.  The  test  lead  ordinarily  bought  from  the  supply 
houses  contains  only  traces  of  silver. 


Ill 


SAMPLING 

PROPER  sampling  is  of  the  utmost  importance,  for  unless  the 
sample  to  be  assayed  accurately  represents  the  lot  of  ore  or 
metallurgic  product  from  which  it  is  taken,  in  other  words,  unless 
it  is  a  true  sample,  the  greatest  care  in  the  assay  itself  means 
nothing.  Large  amounts  of  money  are  involved  in  settlements 
made  on  the  assay  of  final  samples  representing  many  tons  of 
rich  ore,  matte,  bullions,  etc.  Mills  and  smelters  purchase  ores 
by  the  carload  on  the  assay  of  the  final  sample,  and  even  slight 
errors  mean  loss  either  to  the  shipper  or  the  purchaser.  Where 
so-called  "specimen"  assays  are  made,  the  sampling  of  the 
small  amount  of  pulp  is  usually  a  simple  matter,  although  accu- 
racy is  also  required.  In  most  cases  the  samples,  representing 
large  lots,  are  handed  to  the  assayer,  so  that  he  is  usually  not 
directly  concerned  as  to  how  the  samples  were  obtained;  but  in 
general  he  should  be  familiar  as  to  how  sampling  is  conducted. 
Sampling  may  be  classified  under  two  heads: 

1.  Hand  sampling: 

a.  Coning  and  quartering. 

b.  Alternate  shovels. 

c.  Split  shovels. 

d.  Riffling. 

2.  Machine  sampling: 

a.  Part  of  the  ore  stream  for  the  whole  time. 

b.  The  whole  of  the  ore  stream  part  of  the  time. 
Whatever  the  method  of  sampling  used,  a  distinct  relation 

must  exist  between  the  ^weight  of  the  sample  and  the  size  of  the 
ore  particles.  Thus,  if  the  ore  particles  are  large  (10  to  12  in. 
diameter),  a  large  sample  must  be  taken;  if  the  particles  are 
small  (o.io  to  0.20  in.),  a  small  sample  will,  if  properly  taken, 

28 


SAMPLING 


29 


accurately  represent  the  lot  of  ore.1  An  old  rule  in  force  on 
Gilpin  County,  Colorado,  ores,  carrying  from  i  to  4  oz.  gold, 
illustrates  this: 

Diam.  of  largest  piece,  in  inches.  .  .0.04  0.08  0.16  0.32  0.64  1.25  2.50 
Minimum  weight  of  sample,  in  Ibs..  .0.0625  0.50  4  32  256  2048  16348 

The  proper  weight  of  sample  for  any  desired  size  of  ore  particle 
is  obtained  by  multiplying  the  known  weight  for  the  given  size 
by  the  cube  of  the  ratio  of  the  desired  size  to  that  of  the  given 
size. 

As  an  example  of  mill  practice  by  machine  sampling  on 
Cripple  Creek  ores  of  from  2  to  6  oz.  gold  per  ton,  the  following 
is  given : 

The  ore  is  crushed  to  pass  a  i.yin.  ring,  and  from  the  total 
bulk  a  Vezin  sampler  cuts  out  one-fourth.  This  is  passed  to 
crushing  rolls,  which  reduce  it  to  o.2^-in.  size.  It  is  then  elevated 
to  another  Vezin  sampler,  which  takes  out  one-tenth  of  the  bulk, 
the  final  sample  being  one-fortieth  of  the  ore,  or  2.5  per  cent. 
This  is  then  cut  down  and  crushed  finer  and  sampled  in  the 
usual  way  (alternate  shovels,  etc.),  described  further  on.  In 
smelting  works,  where  it  is  desirable  to  have  the  product  going 
to  the  furnaces  as  coarse  as  possible,  the  above  method  is  modified 
by  not  crushing  so  fine  and  by  taking  larger  samples;  or  hand 
sampling  is  employed.  The  size  of  the  sample  depends  not  only 
on  the  size  of  the  ore  particles,  but  also  on  the  nature  of  the 
ore.  If  the  values  are  uniformly  distributed,  smaller  samples 
will  do  than  are  necessary  where  they  are  "spotted"  or  irregu- 
larly distributed.  While  machine  sampling,  with  properly  con- 
structed apparatus,  is  largely  in  use,  and  is  most  desirable  when 
applicable,  hand  sampling  may  be  accurately  performed;  it  is 
still  widely  used  by  smelting  plants,  as  it  avoids  crushing  a  large 
part  of  the  ore. 

The  method  of  "coning  and  quartering"  has  been  in  use  for 
many  years,  and  is  still  employed,  but  it  is  being  displaced  largely 
by  the  "alternate-shovel"  method.  Coning  and  quartering, 
unless  carefully  performed,  which  is  difficult  to  do,  is  apt  to  be 
inaccurate.  In  this  method,  the  thorough  mixing  of  the  ore  is 

1  Brunton,  "The  Theory  and  Practice  of  Ore  Sampling,"  in  Trans.  A.  I.  M.  E., 
XXV,  p.  826.  "Notes  on  Sampling,"  in  "Min.  Rep.,"  XLV,  Nos.  7-16  (inclusive). 


30  A   MANUAL  OF   FIRE   ASSAYING 

essential,  and  the  mixing  is  supposed  to  be  effected  by  coning. 
The  cone  is  built  up  by  men  moving  around  the  circumference 
of  a  circle  and  shoveling  the  ore  upon  the  point  of  a  cone  formed 
by  the  angle  of  repose  of  the  material  falling  vertically  upon  one 
point.  The  samplers  —  from  4  to  8  men  —  move  so  as  to  be 
always  diametrically  opposite  each  other. 

In  order  to  fix  the  point  of  the  cone,  a  rod  is  driven  into  the 
ground  as  a  guide.  It  is  evident  that  the  shoveling  must  be 
very  conscientiously  done  in  order  to  have  the  ore  distribute 
itself  uniformly  (fines  and  coarse)  over  the  surface  of  the  cone; 
but  this  uniformity  is  essential  to  the  obtaining  of  a  true  sample. 
When  the  cone  has  been  built  up,  it  is  then  pulled  down  by  the 
men  walking  around  the  pile  and  scraping  the  ore  from  the  apex 
to  the  base,  until  a  flat  plaque  of  ore  is  made  about  12  or  18  in. 
thick.  Then,  in  the  form  of  a  cross,  plates  of  iron  are  carefully 
centered  on  the  pile  and  driven  in,  dividing  the  plaque  into 
quarters.  Two  opposite  quarters  are  removed  to  the  bins,  and 
the  other  two,  representing  the  sample,  are  reshoveled  into  a 
cone  and  the  operation  repeated.  The  ore  is  then  recrushed  and 
coned  and  quartered  again,  until  finally  a  sample  of  from  25  to 
30  \bs.  is  obtained.  The  number  of  recrushings  depends  upon 
the  size  of  the  first  sample  and  the  nature  of  the  ore.  The  sample 
is  then  ground  fine  and  prepared  for  the  assay  office  by  cutting 
down  with  a  split  sampler  or  other  approved  device.  The  whole 
process  is  slow  and  laborious.  Three  men  can  handle  from  20  to 
25  tons  of  sample  per  shift  at  a  cost  of  from  45  to  50  cents  per  ton. 

The  Alternate-Shovel  Method.  —  The  fundamental  law  of 
sampling  may  be  stated  thus:  In  order  to  properly  take  a  sample 
of  ore,  it  is  necessary  to  take  the  sample  frequently,  or  in  as 
many  places  as  possible,  and  to  take  the  same  quantity  each 
time  at  regular  intervals.  These  conditions  are  fulfilled  by  the 
"alternate-shovel"  method,  which  is  conducted  as  follows: 

The  ore  from  the  cars  is  dumped  on  a  platform  and  men  with 
the  proper  sized  and  shaped  shovels  put  it  into  the  bins,  taking 
out  for  the  sample  a  certain  number,  dependent  on  the  nature 
and  size  of  the  ore  pieces;  viz.,  nine  shovels  are  thrown  into  the 
bins  and  every  tenth  shovel  is  taken  as  a  sample.  If  the  ore  is 
difficult  to  sample,  sample  shovels  may  be  taken  more  frequently; 
or  if  the  ore  is  uniform,  less  frequently.  It  is  usual  to  cut  out 


SAMPLING  3, 

from  one-fifth  to  one:twentieth  of  the  ore.     The  alternate-shovel 
method  possesses  the  following  advantages: 

1.  It  is  more  reliable  and  accurate  than  coning  and  quartering. 

2.  It  is  cheaper  in  operation. 

3.  It  is  quicker. 

The  "quartering"  and  the  "split-shovel"  methods  are  not 
reliable  and  need  not  be  described. 

At  the  plant  of  the  Standard  Smelting  Company,  at  Rapid 
City,  S.  Dak.,  the  shovel  sample  is  passed  to  a  Blake  crusher 
with  a  9X  15  in.  mouth  opening,  having  an  A  discharge,  so  as  to 
halve  the  crushed  sample.  One  of  the  halves  is  fed  directly  to 
a  pair  of  24*12  in.  rolls,  the  discharge  from  these  being  again 
automatically  halved.  If  a  loo-ton  lot  is  taken  as  a  unit,  the 
sample  at  this  point  is  2.5  tons  (taking  every  tenth  shovel),  with 
no  particle  larger  than  0.375  m-  m  diameter.  The  rolls  discharge 
directly  upon  a  plate-iron  floor,  where  the  ore  is  reshoveled, 
every  fifth  to  tenth  shovel  being  taken  as  a  sample,  which  now 
amounts  to  500  or  1000  Ibs.  This  is  put  through  a  pair  of  12  x 
12  in.  sampling  rolls  and  crushed  fine,  and  then  sampled  by  a 
large  Jones  split  or  riffle  sampler,  which  takes  halves,  until  finally 
a  sample  of  between  15  and  20  Ibs.  is  arrived  at.  This  is  put 
through  a  small  cone  grinding  mill,  and  after  a  determination  of 
moisture  on  the  sample  floor  is  sent  to  the  assay  office.  Here 
it  is  cut  down  to  about  2  Ibs.  by  a  small  Jones  sampler,  and  then 
crushed  on  a  buck  board  to  pass  a  i2o-mesh  screen,  furnishing 
the  assay  sample.  This  sample  is  supposed  to  contain  no  mois- 
ture, as  this  was  eliminated  on  the  sample  floor,  where  the  per- 
centage of  moisture  is  determined;  but  as  all  settlements  are 
made  on  dry  samples,  the  final  assay  sample  is  again  heated  at 
120°  C.  for  some  time  in  order  to  expel  any  moisture  which  the 
sample  may  have  absorbed  in  its  passage  from  the  sampling 
works  to  the  assay  office.  The  assay  sample  is  divided  into 
4  parts  and  put  in  paper  sacks.  One  part  is  assayed  by  the 
seller  of  the  ore  or  product;  one  part  by  the  purchaser;  a  third 
part  is  kept  for  emergency;  and  a  fourth  part  is  laid  aside  for  an 
umpire  assay,  if  such  becomes  necessary. 

The  assays  made  by  the  seller  of  the  ore  and  those  made  by 
the  purchaser  of  the  ore  are  called  "control  assays."  If  the 
seller  and  purchaser  agree  within  a  certain  limit,  depending  on 


32  A  MANUAL  OF   FIRE  ASSAYING 

the  value  of  the  ore,  settlement  is  made  on  the  purchaser's  assay, 
or  sometimes  on  the  average  of  the  two  assays.  If  they  do  not 
agree,  it  is  the  practice  for  the  buyer  and  seller  to  reassay  their 
own  samples  or  to  exchange  pulp  samples  and  reassay.  If  they 
do  not  then  agree,  an  umpire  assayer  is  chosen  who  makes  an 
umpire  assay,  by  the  results  of  which  all  parties  abide,  and  on 
which  settlement  is  made.  The  party  that  is  farthest  away 
from  the  result  of  the  umpire  has  to  pay  for  the  assay. 

Controls  are  made  with  three  check  assays,  and  umpires  with 
four  check  assays.  In  sampling  small  lots  in  the  laboratory 
and  cutting  down  for  the  assay  sample,  the  principles  already 
enumerated  also  apply.  Riffle  samplers  are  commonly  used  as 
well  as  the  coning  and  quartering  method,  although  this  last  is 
not  recommended,  even  for  small  lots.  The  final  pulp  sample 


FIG.  30.  —  JONES  RIFFLE  SAMPLER 

is  put  through  a  100-  or  i2O-mesh  screen;  for  high-grade  material, 
150-  to  2OO-mesh  is  better.  It  is  then  thoroughly  mixed  on  a 
rubber  sheet  or  on  heavy  glazed  paper,  spread  out  in  a  thin, 
broad  plaque  0.25  in.  thick,  and  small  lots  taken  with  a  spatula 
at  regular  intervals,  until  the  required  weight  is  obtained.  Fig. 
30  shows  the  Jones  riffle  sampler  and  Fig.  3 1  the  umpire  mechan- 
ical ore  sampler. 

Great  care  should  be  taken  to  clean  all  sampling  apparatus  after 
sampling  each  lot,  so  as  to  avoid  "salting"  samples.  This  also 
applies  to  all  the  crushing  machinery  employed  in  the  sampling. 

Sampling  Lead  Bullion.1  —  Lead  bullion  is  molded  into  bars 

1  G.  M.  Roberts,  "Experiments  in  the  Sampling  of  Silver  Lead  Bullion,"  in 
Trans.  A.  I.  M.  E.,  XXVIII,  p.  413.  Edward  Keller,  "The  Distribution  of  the 
Precious  Metals  and  Impurities  in  Copper,"  ibid.,  XXVII,  p.  106. 

NOTE.  —  For  a   complete   discussion   of  machine   sampling,   consult   A.   W. 


SAMPLING 


33 


of  approximately  80  Ibs.  weight  and  shipped  in  this  form.  The 
best  method  of  sampling  is  to  take  dip  samples  at  regular  intervals 
while  a  lot  of  bars  are  being  molded  at  the  furnace.  When  the 
solid  bars  are  to  be  sampled,  about  the  only  reasonably  accurate 
method  is  to  take  a  "saw  sample."  This  is  carried  out  as  follows: 


FIG.  31. —  UMPIRE  ORE  SAMPLER 

Out  of  a  lot  of  bars,  every  fifth  or  tenth  bar  is  sawed  across  the 
middle  into  two  pieces.  The  saw  dust  is  then  further  cut  down 
to  the  proper  amount  for  sample  and  assayed. 

Copper  bullion  is  sampled  in  a  similar  manner. 

Chip  or  gouge  samples  are  almost  invariably  inaccurate. 

Warwick,  "Notes  on  Sampling,"  published  by  the  Industrial  Publishing  Co. 
Denver,  Colorado  (50  cents). 


IV 


WEIGHING;   BALANCES  AND  WEIGHTS 

THE  balance  used  in  weighing  the  minute  quantities  of  gold 
and  silver  is  a  delicate  piece  of  apparatus  and  must  be  carefully 
adjusted  and  handled  in  order  to  give  accurate  results.  The 
balance  should  be  set  upon  a  firm  foundation,  not  subject  to 
vibration;  otherwise  it  is  apt  to  be  frequently  thrown  out  of 
adjustment.  Stone  or  concrete  piers  set  some  distance  into  the 
ground  and  free  from  the  floor  are  the  best  foundations,  when 
the  vibrations  induced  by  moving  machinery  are  absent.  Where 
such  vibrations  occur,  insulated  shelf  supports  should  be  used. 

Construction.  —  The  balance-beam  is  made  of  aluminum,  gold- 
plated  brass,  special  silver-aluminum  alloys,  etc.,  and  as  light  as 
possible  consistent  with  the  requisite  strength.  The  material 
from  which  it  is  made  should  be  non-magnetic,  and  have  a  small 
coefficient  of  expansion,  so  that  temperature  changes  will  have 
but  slight  effect  on  the  length  of  the  beam.  The  pan-hangers 
are  frequently  of  a  nickel-silver  alloy,  or  of  german  silver,  and 
the  pans  of  aluminum.  The  standards  and  other  metal-work  are 
best  made  of  gold-plated  brass.  The  knife-edges  and  the  plates 
on  which  they  rest  are  made  of  agate,  accurately  polished  and 
ground  true.  The  balance-beam  has  three  knife-edges,  which 
should  be  in  line  in  the  same  plane  in  order  to  give  equal 
sensibility  with  varying  loads.1  The  two  balance-arms,  or  the 
distance  from  the  central  knife-edge  to  each  of  the  outer  knife- 
edges,  should  be  equal  in  length.  This  can  never  be  absolutely 
accomplished,  but  may  be  very  closely  approximated.  The 
accompanying  illustration  (Fig.  32)  shows  the  essential  features 
of  the  balance. 

1  Gottschalk,  "The  Balance,"  in  "West.  Chem.  and  Met.,"  Vol.  II,  April,. 
May  and  June,  1906. 

34 


WEIGHING;   BALANCES  AND  WEIGHTS 


35 


FIG.  32.  —  DIAGRAM  or  ASSAY  BALANCE 

A   the  central  knife-edge. 

B,  B' the  outer  knife-edges. 

D adjustment  for  center  of  gravity  of  the  balance  system. 

C,  C' adjustments  for  equal  moment  of  arms. 

c.  g center  of  gravity  of  the  balance  system. 

Y pointer-arm. 

x distance  of  deflection  of  center  of  gravity,  or  the  gravity  lever-arm. 


36  A  MANUAL  OF   FIRE  ASSAYING 

yf lever-arm  of  small  weight  mf  in  pan. 

mf small  weight. 

M mass  of  balance  system. 

When  the  small  weight  m'  is  put  into  the  pan  it  will  cause  a 
deflection  of  the  pointer,  and  the  center  of  gravity  of  the  balance 
system  shifts.  The  condition  of  equilibrium  is  then  expressed 
by  the  equation 

MX  =  m'x' 

From  this  it  follows  that  if  M  increases,  that  is,  if  the  mass 
of  the  balance  system  becomes  greater,  and  other  conditions 
remain  constant,  the  weight  m'  must  increase  to  cause  the  same 
deflection;  i.e.,  the  sensibility  of  the  balance  will  be  lessened,  the 
sensibility  being  the  amount  of  deflection  caused  by  a  given 
mass.  Assay  balances  must  show  a  sensibility  of  at  least  one-half 
division  of  the  scale  with  a  weight  of  o.oi  mg.  or  even  0.005  mg- 
This  shows  the  necessity  for  an  extremely  light  construction,  or 
a  small  mass  of  the  balance  system.  It  is  evident  from  the 
equation  that  the  sensibility  might  be  preserved  by  increasing 
xf,  i.e.,  by  lengthening  the  arms  of  the  balance;  in  practice, 
however,  this  would  also  very  materially  increase  M,  so  that  the 
gain  is  more  apparent  than  real.  Long  arm  balances  are  also 
very  slow  of  vibration.  Formerly,  long  arm  balances  were  com- 
mon, but  in  modern  assay  balances  the  arm  rarely  exceeds  2.5  in. 

From  the  equation  it  also  follows  that,  if  the  center  of  gravity 
of  the  balance  system  is  placed  at  A,  the  knife-edge  x  becomes 
zero,  and  we  have 

M  X  o  =  m'x' ;  or  m'x'  =  o ; 

or  m'  approaches  o  for  x'  is  practically  constant;  in  other  words, 
thev  balance  will  become  extremely  sensitive,  an  infinitely  small 
weight  in  the  pan  causing  rotation.  While  the  balance  would 
be  very  sensitive,  it  would  also  be  very  unstable  and  "  cranky." 
The  screw-ball  D  is  provided  to  adjust  the  center  of  gravity, 
which  should  be  somewhat  below  the  knife-edge  A.  The  centqf 
of  gravity  is  adjusted  so  that  a  weight  of  o.oi  mg.  in  the  pan  or 
on  the  beam  will  cause  a  deflection  of  from  one-half  to  one  divi- 
sion of  the  pointer.  The  lower  the  center  of  gravity  of  the  balance 
system,  the  more  rapid  the  oscillation  of  the  balance.  The 


WEIGHING;   BALANCES  AND   WEIGHTS  37 

higher  or  the  nearer  the  point  of  suspension,  the  slower  the  oscil- 
lations and  the  greater  the  sensibility. 

Before  weighing,  the  balance  is  always  thoroughly  cleaned  in 
every  part  from  dust  by  a  soft  camel's-hair  brush,  made  perfectly 
level  by  adjusting  the  leveling  screws,  and  the  pointer  stand- 
ardized to  o  by  the  little  thumb-screws  C,  O.  To  do  this,  the 
balance  is  set  in  motion  until  the  pointer  swings  to  from  5  to  8 
divisions  on  the  scale  each  side  of  the  zero  mark.  If  the  balance- 
arms  are  equal  in  moment,  the  pointer  will  swing  practically  an 
equal  number  of  divisions  on  each  side,  losing,  however,  a  trifle 
on  each  swing,  thus:  +8, -7.75, +  7.5, -7.25, +  7, -6.75,  etc.,  the 
loss  being  due  to  friction  and  a  gradual  settling  back  into  equi- 
librium. If  the  swings  are  not  as  outlined,  the  adjustment  is 
made  until  they  become  so.  The  balance  is  then  tested  for 
sensibility  as  described,  and  the  adjustment  made  for  it,  if  neces- 
sary, by  moving  the  center  of  gravity.  If  the  balance-arms  are 
suspected  of  being  unequal  in  length  (though  this  is  rare  in  good 
balances),  weighing  by  "substitution,"  or  double-weighing,  is 
adopted.  In  this  method,  the  object  to  be  weighed  is  placed 
first  in  one  pan  and  weighed,  and  then  in  the  other,  the  true 
weight  being  the  square  root  of  the  product  of  the  two  weights 
found.  When  the  sensibility  of  the  balance  is  accurately  known, 
no  adjustment  for  equal  moment  of  arms  need  be  made,  but 
weighing  may  be  done  by  deflection,  after  the  true  zero  or  equi- 
librium point  is  found.  This  is  found  as  follows:  Start  the 
balance  swinging  and  count  swings  to  the  left  as  minus  and  to 
the  right  as  plus.  Suppose  the  swings  are  as  follows:  —8,  +3, 
—  7.5,  The  zero-point  then  is 

f2(~7'5)  (  +  3)  =  -475  (divisions). 

Then  place  the  particle  to  be  weighed  on  the  right-hand  pan 
and  weigh  again.  The  swings  are  as  follows:  —  10,  +2,  —9.5. 
The  sensibility  of  the  balance  being  0.5  deflection  for  each  o.oi  mg., 
the  weight  of  the  particle  will  be 

(-4.75)=   -  3  (divisions). 


Sensibility,     i     div.  =  0.02    mg.     The    weight    is    therefore 
3  X  0.02  =  0.06  mg. 


38  A  MANUAL  OF   FIRE  ASSAYING 

This  method,  however,  is  not  generally  to  be  recommended; 
the  "rider"  should  be  used  for  the  determination  of  the  fractional 
parts  of  the  milligram.  The  balance  should  also  be  adjusted  for 
equal  moment  of  arms,  as  described,  before  weighing. 

In  order  to  detect  inequality  in  the  length  of  the  arms,  stand- 
ardize the  balance  to  the  true  zero,  place  a  i-gram  weight  on  the 
right  pan,  and  an  old  or  worn  i-gram  weight  on  the  left  pan, 
and  bring  the  balance  into  approximate  equilibrium  by  adding 
minute  quantities  of  old  rider-wire  to  the  short  weight. 

Let  the  gram  weight  in  the  right  pan  be  called  A. 

Let  the  counterpoise  in  the  left  pan  be  called  B. 

Let  R  be  the  right  lever-arm  and  L  the  left  lever-arm. 

Determine  the  zero-point  of  the  balance  in  the  manner  de- 
scribed. If  this  zero-point  differs  from  that  of  the  unloaded 
balance,  bring  the  balance  to  the  old  zero-point  by  moving  the 
rider  on  the  left  or  right  arm,  as  required. 

Let  the  weight  indicated  by  the  rider  be  called  +  m  or  —  m, 
as  it  may  act  with  or  against  B  to  bring  the  balance  system 
back  to  the  original  zero-point. 

Now  shift  the  weight  A  to  the  left  pan  and  B  to  the  right 
pan;  remove  the  rider  and  again  determine  the  zero-point,  and 
then  manipulate  the  rider  to  bring  the  balance  system  to  the 
zero-point  of  the  unloaded  balance  and  call  the  weight  indicated 
by  the  rider  ±  n,  as  it  may  act  with  or  against  A.  The  following 
equations  will  then  result: 

1.  AR=(B±m)L  A  =  (B±m)-g 

2.  BR  =  (A±n)L  B  =  (A±n)~ 


JR       R      (B  +  A±m±n] 

If  m  =    —n,  or  the  reversal  of  the  masses  shifts  the  zero- 
point  exactly  as  much  to  one  side  as  it  was  before  on  the  other 

of  the  actual  o,  the  balance  has  equal  arms;  i.e.,  -„=  i.    75  should 

K  K 

not  exceed  i  ±0.000003. 


WEIGHING;    BALANCES  AND  WEIGHTS 


39 


Some  assayers  weigh  by  "no  deflection."  They  adjust  the 
balance  to  the  true  zero,  place  the  bead  to  be  weighed  in 
the  right-hand  pan,  and  then  by  the  addition  of  weights  and  the 
moving  of  the  rider  by  repeated  trials,  balance  the  bead,  so  that 
finally,  when  the  balance  is  lowered  gently  on  its  knife-edge,  no 
deflection  of  the  pointer  takes  place.  This  method,  however,  is 
not  recommended,  as  it  disregards  friction  and  inertia,  and  for 
small  weights  gives  inaccurate  results.  Care  must  be  taken  to 


FIG.  33. —  PULP  BALANCE 

have  an  even  temperature  in  the  balance  room,  preferably  about 
60°  F.  The  balance  must  not  be  exposed  to  a  source  of  heat 
which  will  radiate  unsymmetrically,  otherwise  unequal  expansion 
of  balance-arms  will  cause  incorrect  weights.  In  weighing,  the 
balance-door  should  always  be  closed  to  avoid  the  disturbing 
effect  of  slight  air  currents.  The  true  weight  of  a  mass  can  be 
determined  only  by  correcting  for  the  buoyant  effect  of  air.  The 
error,  however,  is  so  small  that  it  may  ordinarily  be  neglected.1 
Pulp  and  reagent  balances  are  shown  in  Fig.  33.  A  modern 
assay  button  balance  is  shown  in  Fig.  34. 

1  Ostwald,  " Physico-Chemical  Measurements,"  1894,  p.  38.     Ames  and  Bliss, 
"A  Manual  of  Experiments  in  Physics,"  p.  151. 


A   MANUAL  OF   FIRE   ASSAYING 


FIG.  34.  —  ASSAY  BUTTON  BALANCE 

Weights.  —  The  weights  used  in  weighing  beads  are  milligram 
weights,  usually  from  i  mg.  up  to  1000  mgs.,  the  units  being  as 
follows:  i,  2,  5,  10,  20,  50,  100,  200,  500  and  1000  mgs.  They 
are  best  made  of  platinum,  as  the  material  must  be  non-corrosive, 
so  that  the  weight  will  remain  constant.  Riders  are  used  to 
determine  weights  up  to  i  mg.  the  balance-beams  being 
divided  into  100  equal  spaces,  each  space  being  equivalent  to 
o.oi  mg.  with  a  i-mg.  rider.  Riders  are  made  of  fine  platinum 
wire,  and  for  assay  balances  usually  come  as  0.5-  and  i-mg. 
riders.  One-milligram  riders  are  commonly  used.  Where  the 
balance  can  readily  be  made  sensitive  to  0.005  mg->  a5~mg-  riders 
can  be  used  with  profit;  otherwise  i-mg.  riders  are  preferable,  as 
they  are  not  so  readily  injured  by  handling.  Riders  are  fre- 
quently sold  which  are  not  of  true  weight,  and  it  is  essential  to 
check  them  before  using.  The  same  is  true  of  weights.  It  is 
desirable  for  every  assay  office  to  have  a  set  of  standardized 
weights  for  comparison.  These  standardized  weights  can  be 


WEIGHING;   BALANCES  AND  WEIGHTS  41 

purchased  from  the  balance  firms;  or  a  set  may  be  corrected  by 
the  Government  Bureau  of  Standards. 

The  Assay-Ton  System.  —  Gram  and  assay-ton  weights  are 
used  to  weigh  pulp  and  fluxes.  The  assay-ton  system  was  devised 
by  Professor  Charles  F.  Chandler,  of  Columbia  University,  New 
York,  and  reconciles  the  difficulties  arising  from  the  fact  that  all 
ores,  etc.,  are  weighed  by  the  avoirdupois  system,  while  precious 
metals  are  weighed  by  the  troy  system.  The  basis  of  the  assay 
ton  is  the  number  of  troy  ounces  in  i  ton  (2000  Ibs.)  avoirdupois. 

i  ton  =  2000  Ibs.; 

i  Ib.  (avoirdupois)  =  7000  troy  grains; 
therefore,    i  ton  =  14,000,000  troy  grains, 

i  oz.  (troy)    =  480  grains; 

f         14,000,000 
therefore,  -   — ~ =  29,166  oz.  (troy). 

Then,  taking  i  mg.  as  the  unit,  i  assay  ton  =  29,166  mgs., 
or  29.166  grams,  and  i  mg.  bears  the  same  relation  to  i  assay 
ton  as  i  oz.  troy  bears  to  i  ton  of  2000  Ibs.  avoirdupois. 

From  this  it  follows  that  if  i  assay  ton  of  ore  is  tfeken,  and 
the  silver  and  gold  from  this  is  weighed  in  milligrams,  this  weight 
will  represent  ounces  troy  per  ton  of  ore.  Fig.  35  shows  a  set  of 
platinum  assay  weights;  Figs.  36  arid  37  show  a  set  of  assay-ton 
and  gram  weights,  respectively. 


A  MANUAL  OF   FIRE  ASSAYING 


FIG.  35.  —  Platinum  Assay  Weights 


FIG.  36.  —  Assay  Ton  Weights 


FIG.  37.  —  Gram  Weights 


REDUCTION  AND  OXIDATION   REACTIONS 

A  REDUCTION  reaction,  as  particularly  defined  for  assaying,  is 
one  in  which  a  metal  is  reduced  from  its  compounds  by  some 
reducing  agent.  The  chemical  definition  is  also  applicable  in 
that,  in  assaying,  we  frequently  reduce  a  compound  from  a  state 
of  higher  oxidation  to  a  lower  state  of  oxidation  by  means  of  a 
reducing  agent. 

An  oxidation  reaction  is  one  in  which  a  metal  or  a  compound 
is  changed  to  a  compound  of  a  higher  state  of  oxidation;  for 
example,  Pb  to  PbO,  S  to  SO2,  or  PbO  to  PbO2.  Reduction  and 
oxidation  reactions  frequently  occur  in  assaying,  and  it  is  essential 
that  the  assayer  be  thoroughly  familiar  with  the  theory  and 
facts.  In  speaking  of  reducing  agents  and  reduction  with  special 
reference  to  assaying,  we  have  chiefly  in  mind  such  reagents  as 
reduce  metallic  lead  from  litharge  in  the  crucible.  The  chief  of 
these  are:  (i)  argol,  (2)  charcoal  or  coke  or  coal  dust,  (3)  flour  or 
sugar.  These  are  added  to  the  charge  in  sufficient  quantity  to 
produce  the  proper  size  of  lead  button  in  the  crucible  assay. 
It  often  happens  that  an  ore  will  contain  reducing  agents,  chiefly 
sulphides,  so  that  it  becomes  unnecessary  to  add  an  extraneous 
agent.  In  fact,  it  may  contain  an  excess  of  reducing  agent, 
requiring  an  oxidizing  agent  to  destroy  the  excess. 

The  reduction  of  lead  by  argol  is  expressed  by  the  following 
equation : 

10  PbO  +  2KHC4H4O6  -  loPb  +  5H2O  +  K2O  +  8CO2 
376  2070 

One  gram  of  argol  will  reduce  5.50  grams  of  lead  from  5.93  or 
more  grams  of  PbO.  The  above  formula  for  argol  is  that  of  pure 
bitartrate  of  potassium.  Argol  contains  as  impurity  a  certain 
amount  of  carbonaceous  matter,  so  that  its  reducing  power  will 

43 


44  A  MANUAL  OF   FIRE  ASSAYING 

be  increased.  It  will  be  found  that  the  actual  reducing  power 
of  i  gram  of  argol  varies  between  7  and  9.5  grams  of  lead,  depend- 
ent on  the  argol  used. 

The  reduction  of  lead  by  charcoal  is  expressed  by  the  following 
reactions  : 

2PbO  +  C  =  2Pb  +  C02 
12  414 

One  gram  of  carbon  will  reduce  34.5  grams  of  Pb.  As  char- 
coal, coal  or  coke  dust  will  contain  more  or  less  inert  ash  which 
has  no  reducing  effect,  the  actual  amount  of  lead  reduced  will  be 
materially  less.  It  will  usually  be  found  to  range  between  20 
and  30  grams  per  gram  of  carbonaceous  reducing  agent  used. 

Flour  will  reduce  from  9  to  12  grams  of  lead  per  gram,  de- 
pending on  the  nature  of  the  flour. 

The  common  sulphides  most  frequently  found  in  ores,  and 
which  give  the  ores  containing  them  reducing  powers,  are  :  Pyrite 
(FeS2),  pyrrhotite  (Fe7S8),  arsenopyrite  (FeAsS),  chalcopyrite 
(CuFeS2),  chalcocite  (Cu2S),  stibnite  (Sb2S3),  galena  (PbS),  and 
sphalerite  (ZnS). 

The  amount  of  lead  reduced  per  gram  of  the  respective  sul- 
phides varies  according  to  the  combination  of  conditions,  which 
will  be  fully  discussed. 

Taking  pyrite  as  an  example,  the  following  equation  expresses 
the  reaction  which  takes  place  when  it  is  fused  with  soda  and 
litharge: 

(a)  2FeS2  +  i5PbO  -  Fe2O3  +  4S 

240  3105 

(b)  4SO3  +  4Na2CO3  =  4Na2SO4 


One  gram  of  pure  pyrite  reduces  12.9  grams  of  lead.  The 
result  can  readily  be  obtained  by  the  following  charge: 

Pyrite  ...............................     3  grams 

NaHCO.3  ............................    10  grams 

PbO    ...............................  100  grams 

The  result  could  not  be  obtained  were  the  pyrite  to  be  fused 
with  litharge  alone,  as  the  presence  of  soda,  a  strongly  alkaline 
base,  induces  the  formation  of  sulphuric  anhydride  (SO3),  which 
combines  with  soda  to  form  sodium  sulphate  (Na2SO4).  This 
sodium  sulphate  will  float  on  top  of  the  slag  and  is  not  decom- 


REDUCTION  AND  OXIDATION   REACTIONS 


45 


posed  by  the  temperature  usually  attained  in  the  muffle.  It 
separates  out  on  cooling  as  a  fused  white  mass.  Its  melting- 
point  is  86 1  °  C.  When  the  oxidizing  action  in  the  above  charge 
is  diminished  by  decreasing  the  litharge  1  to  below  70  grams, 
the  iron  is  only  partially  oxidized  to  the  ferric  condition  and  the 
two  following  equations  express  the  reactions:2 

FeS2  +  yPbO  =  FeO  +2SO3  +  yPb 
2FeS3  +  i5?bO  =-Fe2O3  +  4SO3  +  ^Pb 

The  first  equation  will  give  12  grams  of  Pb  per  gram  of  pyrite, 
and  the  second  will  give  12.9  grams.  The  accompanying  table 
gives  the  reducing  powers  of  the  various  substances  as  determined 
by  the  litharge-soda  charge  given  for  pyrite. 

TABLE   I. —  REDUCING   POWERS   OF  AGENTS 


NAME  OF  REDUCING  AGENT 

QUANTITY  OF  LEAD  IN  GRAMS 
REDUCED  BY  I  GRAM 
OF  REDUCING  AGENT 

Argol 

9.61 

Flour 

IO.s3 

Sugar  
Charcoal                                                           

Il.yS 
26.0 

Sulphur                                                                 .  . 

i8.ii       (See  Table   II) 

Pyrite   
Pyrrhotite                  

12.24 
8.71 

Stibnitc 

7.17 

Chalcocite 

4.38 

Sphalerite  

8.16 

When  no  soda  is  present  to  induce  the  formation  of  alkaline 
sulphates,  the  following  reaction  takes  place,  sulphur  dioxide 
(SO2)  being  formed : 

FeS,  +  5PbO  =  FeO  +  2SO2  +  5Pb; 

120  1035 

or  i  gram  of  pyrite  reduces  8.6  grams  of  lead. 

1  E.  H.  Miller,  "The  Reduction  of  Lead  from  Litharge,"  in  Trans.  A.  I.  M.  E., 
XXXIV,  p.  395. 

2  It  must  be  borne  in  mind  that  while  we  speak  of  a  "reducing"  or  an  "oxi- 
dizing" reaction,  the  reaction  is  really  of  both  natures,  for  while  litharge  is  "re- 
duced," the  iron  pyrite  is  "oxidized." 


46  A   MANUAL  OF   FIRE  ASSAYING 

In  the  assay,  as  ordinarily  performed,  the  foregoing  conditions 
are  modified  by  the  presence  of  other  substances,  in  the  main 
by  silica.  Lead  oxide  readily  forms  silicates  with  silica,  and  the 
mono-,  bi-,  and  tri-sil'icates  are  easily  fusible,  while  those  of  a 
higher  degree  are  fusible  with  difficulty.  When  a  reducing  agent 
(argol,  sulphides,  etc.)  is  fused  with  a  silicate  of  lead,  or  with  a 
charge  containing  litharge  and  silica,  practically  no  lead  is  reduced 
when  the  silica  is  present  in  amounts  to  form  a  trisilicate  or  above, 
and  little  lead  is  reduced  when  the  silica  is  present  in  amounts 
to  form  a  mono-  or  bisilicate.  The  reason  for  this  is  that  the 
silicates  of  lead  are  not  reduced  by  sulphides  or  carbonaceous 
reducing  agents  at  temperatures  below  about  1000°  C.  Above 
that  temperature  reduction  takes  place  more  readily.  The  higher 
the  silicate  degree  the  more  difficult  is  the  reduction.  If,  how- 
ever, certain  other  bases,  such  as  ferrous  oxide  (FeO),  soda 
(Na2O),  or  lime  (CaO),  are  present  (as  is  the  case  with  most 
ores),  reduction  of  lead  from  the  silicate  occurs,  with  ferrous 
oxide  or  soda,  at  a  comparatively  low  temperature;  but  with 
lime  alone,  only  at  a  high  temperature.  The  following  equation 
expresses  this  condition : 

Pb2SiO4  +  2  FeO  +  C  =  Fe2SiO4  +  CO2  +  2?b 

No  difficulty  is  encountered  in  reducing  lead  from  the  borates 
of  lead  and  soda,  by  the  ordinary  reducing  agents,  at  1 100°  C. 
While  soda  influences  the  amount  of  lead  reduced  from  litharge 
by  the  sulphides  present,  it  has  not  that  influence  on  carbonaceous 
reducing  agents,  except  in  so  far  as  it  may  reduce  the  acidity  of 
the  charge  and  thus  favor  reduction. 

The  following  charge  gave  results  as  tabulated  below:1 

Reducing  agent i  gram  Soda  bicarbonate TO  grams 

Litharge 45  grams  Silica 7  grams 

Pyrite,  in  this  table,  shows  a  reduction  of  9.30  grams  of  lead 
per  gram,  a  figure  to  be  expected  when  its  sulphur  goes  off  partly 
as  SO2  and  partly  as  SO3.  If  the  soda  in  the  preceding  charge 
is  increased,  the  lead  button  will  approach  the  maximum  reducible 
by  pyrite. 

1  "The  Reduction  of  Lead  from  Litharge,"  ibid. 


REDUCTION  AND  OXIDATION   REACTIONS  47 

TABLE  II. —  REDUCING   POWER   OF  AGENTS 


NAME  OF  REDUCING  AGENT 


QUANTITY  OF  LEAD  REDUCED 

BY  i  GRAM 
or    REDUCING  AGENT 


Argol 


Flour 

Sugar    : 

Charcoal 

Pyrite 

Sulphur 


9.6 
10.92 
11.74 
26.08 

9-3° 
i8.ii 


NOTE.  —  Compare  Table  I  with  this. 

When  carbonaceous  reducing  agents  are  used  to  obtain  the 
required  lead  button,  the  nature  of  the  charge,  as  regards,  acidity 
(due  to  SiO2  or  borax),  has  little  influence  on  the  size  of  button, 
provided  sufficient  bases,  outside  of  PbO,  are  present  to  decom- 
pose lead  silicates  formed,  and  the  silicate  degree  does  not  exceed 
a  monosilicate.  The  amount  of  litharge  present  has  some  in- 
fluence. The  quantity  of  carbonaceous  reducing  agent  remaining 
constant,  the  size  of  button  will  increase  somewhat  with  increasing 
amounts  of  PbO  in  the  charge.  When  the  reducing  agent  is  a 
sulphide  (often  a  natural  constituent  of  the  ore),  the  acidity  of 
the  charge  influences,  to  a  certain  extent,  the  size  of  button 
obtainable.  It  is,  however,  the  amount  of  alkaline  base  present 
(K2O,  Na2O)  that  exerts  the  most  powerful  influence,  its  presence 
inducing  the  formation  of  SO3  and,  consequently,  sulphates,  thus 
reducing  larger  amounts  of  lead  than  when  no  alkaline  bases  are 
present,  the  sulphur  going  off  as  SO2. 

Oxidation.  —  Oxidation  of  impurities  in  ores  is  frequently 
necessary  in  order  to  obtain  good  results  in  the  assay.  When 
ores  contain  an  excess  of  sulphides,  arsenides,  etc.  (by  an  excess 
is  meant  a  quantity  above  that  which  will  give  the  required  size 
of  lead  button),  an  oxidizing  agent  is  required  to  oxidize  this 
excess,  enabling  it  to  be  volatilized  or  slagged.  Oxidation  of 
impurities  is  accomplished  in  one  of  two  ways. 

1  Due  to  the  ready  distillation  of  sulphur,  this  figure  is  difficult  to  obtain; 
i  gram  of  sulphur  will  usually  reduce  6  or  8  grams  of  lead. 


48  A  MANUAL  OF   FIRE  ASSAYING 

1.  By  the  addition  of  potassium  nitrate  (KNO3)  to  the  charge 
(or  other  oxidizing  agents). 

2.  By  roasting  the  ore,  thus  using  the  oxygen  of  the  air  for 
the  oxidation  of  impurities. 

When  niter  is  added  to  an  assay,  it  reacts  with  the  most 
easily  oxidizable  compound  in  the  charge,  which  is  usually  the 
reducing  agent,  i.e.,  the  sulphide  present.  Extraneous  reducing 
agents,  such  as  argol,  flour,  or  charcoal,  are  present  simulta- 
neously with  niter  only  when  it  is  desired  to  determine  the  oxi- 
dizing power  of  niter  against  these  reagents.  For  the  sake  of 
convenience,  the  oxidizing  power  of  niter  is  expressed  in  terms 
of  lead.  If  finely  divided  lead  is  fused  with  niter,  the  fusion 
reaching  a  temperature  of  1000°  C.  after  one-half  hour,  the  fol- 
lowing reaction  takes  place,  approximately: 

7Pb  +  6KN03  =  yPbO  +  3K20  +  3N2  +4O2; 

or  i  gram  of  niter  oxidizes  2.39  grams  of  lead.  The  actual  number 
of  grams  of  lead  oxidized,  determined  by  a  considerable  number 
of  experiments,  has  been  found  to  be  2.37.  The  analysis  of  the 
gas  caught  from  the  fusion  showed  10.75  per  cent,  oxygen,  the 
balance  being  nitrogen.  Oxides  of  nitrogen  were  absent.  This 
indicates  that  when  niter  is  used  in  the  crucible  fusion,  oxygen 
is  evolved  which,  under  certain  conditions,  may  escape  from  the 
charge  without  reaction.  As  already  stated,  the  niter  will  react 
with  the  reducing  agent;  expressing  its  oxidizing  power  in  terms 
of  lead  is  merely  for  convenience.  In  certain  types  of  charges, 
i.e.,  those  containing  litharge,  niter,  and  reducing  agent,  or 
litharge,  soda,  niter  and  reducing  agent,  practically  theoretical 
results  may  be  obtained;  e.g.,  the  oxidizing  power  of  niter  as 
compared  to  charcoal  is  expressed  by  the  following  equation  : 

4KNO3  +  50  =  2K2O  +  5CO2  +  2N2; 


or  i  gram  of  niter  oxidizes  0.15  gram  of  carbon, 

Taking  the  reducing  power  of  pure  carbon  as  34.5  grams  of 
lead,  the  oxidizing  power  of  niter  against  carbon,  expressed  in 
terms  of  lead,  is  0.15  X  34.5,  or  5.17  grams.  Ten  fusions  of  a 
charge  composed  of  85  grams  PbO,  i  gram  charcoal,  3  grams 
KNO3,  with  5  grams  PbO  as  a  cover,  gave  very  concordant 
results,  and  showed  \he  oxidizing  power  of  niter  to  be  5.10. 


REDUCTION  AND  OXIDATION   REACTIONS  49 

The  reducing  power  of  the  charcoal  was  determined  by  five 
fusions  with  the  same  charge,  omitting  the  KNOg.1  These 
results,  of  course,  can  also  be  obtained  by  an  impure  charcoal; 
for,  taking  one  which  has  a  reducing  power  of  26  grams  of  lead 

(this  was  used  in  the  above  fusions),  it  then  contains  •—  -  or 

34-5 
0.765  gram  pure  carbon.     If  3  grams  of  niter  have  been  added 

to  the  charge,  the  available  carbon  for  reduction  will  be  0.765  - 
(3  xo.15)   or  0.315  gram,  which  will   reduce  34.5  X  0.315,  or 
10.75,  grams  of  lead.     The  oxidizing  power  of  niter  expressed  in 
lead,  then,  is 

26—  10.75 

—  —  '—,  or  5.12  grams. 

Considering  a  sulphide  and  niter,  and  it  is  in  this  connection 
that  niter  is  almost  invariably  used,  the  following  reaction  takes 
place  in  the  litharge-soda  charge  already  mentioned: 

6KNO3  +  2FeS2  =  Fe2O3  +  3SO3  +  3K2SO4  +  3N2 
3SO3  +  3Na2CO3  =  3Na2SO4 


or  i  gram  of  niter  oxidizes  0.39  gram  of  pyrite.  In  the  litharge- 
soda  charge,  i  gram  of  pyrite  reduces  12.22  grams  of  lead;  there- 
fore, i  gram  of  niter  in  this  instance  would  oxidize  12.22  X  0.39, 
or  4.76,  grams  of  lead.  *  The  accompanying  table  2  shows  actual 
results  obtained  for  the  oxidizing  power  of  niter  against  different 
reducing  agents. 

TABLE   III.—  OXIDIZING   POWER   OF  NITER 


REDUCING  AGENT 

OXIDIZING  POWER  OF 
NITER  IN  TERMS  OF  LEAD 

Pyrite 

4-73  grams 

Charcoal 

5.15  grams 

Flour    

5.09  grams 

Argol    .... 

4.76  grams 

It  follows,  therefore,  that  the  oxidizing  power  of  niter  varies 
with  the  reducing  agent  used. 

1  This  finding  confirms  that  of  E.  H.  Miller,  in  Trans.  A.  I.  M.  E.,  XXXIV, 
p.  395.  2  Ibid. 


5o  A  MANUAL  OF   FIRE  ASSAYING 

When  the  assay  charge  contains  silica  and  borax  glass,  the 
above  figures  no  longer  hold,  for  in  their  presence  oxygen  is 
evolved  by  the  niter,  which  escapes  from  the  charge,  as  in  the 
case  of  the  oxidation  of  metallic  lead  by  niter.  The  amount  of 
oxygen  lost  (thus  reducing  the  oxidizing  power  of  niter)  is  prob- 
ably a  function  of  the  rate  of  rise  of  temperature,  but  evidence 
also  points  to  the  fact  that  silica  reacts  with  the  niter,  setting 
free  oxygen,  at  a  temperature  very  close  to  that  at  which  niter 
reacts  with  charcoal,  or  at  which  oxygen  will  react  with  carbon. 
Niter  fuses  at  339°  C,  but  does  not  give  off  oxygen  when  fused 
alone  until  530°  C.  is  reached.  Charcoal  ignites  at  temperatures1 
ranging  from  340°  C.  to  700°  C.,  depending  upon  the  temperature 
at  which  it  was  burnt,  while  silica  begins  to  react  with  niter  at 
very  nearly  450°  C.,  probably  according  to  the  following  reac- 
tion: 

2  KN03  4-  Si02  =  K2Si03  +  50  +  N 


2 


Thus,  during  the  period  in  which  the  temperature  in  the 
crucible  gradually  rises  to  a  yellow  heat  (that  of  the  muffle), 
oxygen  escapes  during  the  range  from  400°  C.  to  500°  C.,  etc., 
this  last  being  taken  as  an  average  temperature  at  which  charcoal 
will  begin  actively  to  oxidize.2 

Niter  will  begin  to  react  with  argol  and  pyrite  at  practically 
its  melting-point. 

The  oxidizing  power  of  niter  against  charcoal  in  charges 
containing  silica  will  frequently  vary  between  3.7  and  4.2  grams 
of  lead,  averaging  about  4  grams.  This  is  i.i  grams  lower  than 
in  the  litharge-soda  charge.  The  oxidizing  power  of  niter  against 
sulphides  is  but  little  lowered  by  the  presence  of  silica  or  borax 
glass.  When  the  oxidizing  power  of  niter  against  pyrite  (sul- 
phides) is  considered,  and  expressed  in  terms  of  lead,  the  varying 
reducing  power  of  sulphides  in  different  charges  has  to  be  taken 
into  account.  Taking  as  an  example  a  charge  containing  con- 
siderable silica,  so  that  a  large  part  of  the  soda  (alkaline  base)  is 
absorbed  as  a  silicate,  leaving  but  little  to  form  sulphate  from 
the  oxidation  of  the  pyrite,  it  is  found  that  the  reducing  power 

1  From  a  number  of  experiments  by  the  author,  willow  charcoal  was  found 
to  begin  reaction  with  niter  at  very  close  to  440°  C. 

2  This  is  offered  tentatively,  as  an  explanation  of  what  occurs. 


REDUCTION   AND  OXIDATION   REACTIONS  51 

of  pyrite  is  9  grams  of  lead,  as  already  noted.  In  this  charge, 
niter  will  react  with  pyrite  as  follows: 

4FeS2  +  ioKNO3  =  4FeO  +  5K2SO4  +  ^SO2  +  5N2; 

or  i  gram  of  niter  oxidizes  0.475  gram  pyrite.  The  oxidizing 
power  of  niter  expressed  in  lead  is  then  9  X  0.475,  or  4-275 
grams.  Actually,  it  will  be  very  little  lower  than  this,  as  but 
little  oxygen  escapes  without  action.  The  actual  figure  obtained 
by  experiment  is  very  close  to  4.20. 

It  is  evident  from  this  that  the  oxidizing  power  of  niter  varies 
with  the  type  of  charge  used.  It  ranges,  for  pyrite,  from  about 
4  grams  in  acid  charges  to  4.76  in  basic  charges  (containing  no 
silica).  It  varies  still  more  with  other  sulphides.  It  has  been 
the  practice  of  assayers  in  making  the  niter  fusion  to  run  a  pre- 
liminary assay  in  a  comparatively  basic  charge  (approximately 
the  litharge-soda  type),  and  use  the  figure  obtained  for  the  re- 
ducing power  of  the  ore  in  this  charge  in  calculating  the  amount 
of  niter  for  the  final  fusion,  usually  made  quite  acid.  In  this 
way  discordant  results  are  obtained,  for  both  the  reducing  power 
of  the  ore  and  the  oxidizing  power  of  niter  vary  in  the  different 
charges. 

Supposing  that  the  preliminary  assay  showed  the  reducing 
power  of  a  nearly  pure  pyrite  to  be  12  grams  of  lead  per  gram 
of  ore.  Using  a  0.5  assay  ton  in  the  final  fusion,  on  this  basis 
the  amount  of  lead  reduced  would  be  12  X  15,  or  180  grams. 
Subtracting  the  weight  of  the  lead  button,  20,  from  this  leaves 
-the  equivalent  of  160  grams  of  lead  to  be  oxidized.  Taking  4 
as  the  oxidizing  power  of  niter  in  the  final  charge,  40  grams  of 
niter  would  be  added.  But  in  the  final  charge,  owing  to  its 
acidity,  the  reducing  power  of  the  pyrite  is  but  10  grams  of  lead 
per  i  gram  of  ore,  and  the  total  reducing  power  of  J  assay  ton 
is  150  grams.  It  therefore  follows  that  the  final  result  will  show 
no  button.  The  oxidizing  power  for  niter  which  should  have 
been  used  is  -y*  X  4,  or  5.3,  and  31  grams  of  niter  added.  This, 
then,  would  give  approximately  the  proper  sized  button.  As 
the  range  of  reducing  power  for  pyrite  is  from  about  9  to  12.2 
grams  of  lead,  according  to  whether  the  charge  is  acid  and  con- 
tains little  soda,  or  is  of  the  litharge-soda  type,  the  most  satis- 
factory way  to  determine  the  amount  of  niter  to  add  is  to  have 


52  A   MANUAL  OF   FIRE  ASSAYING 

the  nature  of  the  preliminary  charge  the  same  as  that  of  the 
final  charge,  and  then  use  the  figure  4  to  4.2  as  the  oxidizing 
power  of  niter.1  The  following  charges  are  recommended  to 
determine  oxidizing  and  reducing  powers: 

PRELIMINARY  ASSAY,  No.  i  PRELIMINARY  ASSAY,  No.  2 

$  grams  of  pyritous  ore  $  grams  of  pyritous  ore 

8  grams  of  SiO2  8  grams  of  SiOs 

100  grams  of  PbO  100  grams  of  PbO 

12  grams  of  Na^COa  12  grams  of  Na^COa 

Borax  glass  cover  3  grams  of  KNOa 

Borax  glass  cover 

The  difference  in  weight  of  the  lead  buttons  of  preliminary 
assays  Nos.  i  and  2,  divided  by  3,  will  give  the  oxidizing  power 
of  niter  in  the  type  of  charge  used.  The  weight  of  the  button  of 
preliminary  assay  No.  i,  divided  by  5,  gives  the  reducing  power 
of  the  ore. 

PRELIMINARY  ASSAY,  No.  3 

5  grams  of  pyritous  ore  12  grams  of  NaoCOa 

100  grams  of  PbO  Salt  cover 

It  will  be  noticed  that  the  reducing  power  of  the  ore  is  greater 
than  that  obtained  in  preliminary  assay  No.  i.  In  order  to 
determine  the  reducing  power  of  argol  and  charcoal,  make  up  the 
following  charges  in  duplicate: 

PRELIMINARY  ASSAY,  No.  4  PRELIMINARY  ASSAY,  No.  5 

5  grams  SiOo  5  grams  SiOa 

60  grams  PbO  60  grams  PbO 

10  grams  NaoCOs  10  grams  Na2COa 

2  grams  argol  i  gram  charcoal  or  coke  or 

Borax  glass  cover  coal  dust 

Borax  glass  cover 

In  order  to  determine  the  oxidizing  power  of  niter  as  com- 
pared to  charcoal,  make  up  the  following  charge  in  duplicate: 

PRELIMINARY  ASSAY,  No.  6 

5  grams  SiO2  i  gram  charcoal,  etc. 

60  grams  PbO  3  grams  KNOa 

10  grams  Na2COa  Borax  glass  cover 

Calculate  results  as  directed  for  niter  in  pyritous  ores. 
Certain  basic  ores  will  have  an  appreciable  oxidizing  power, 

1  This  has  reference  to  pure  dry  KNOa. 


REDUCTION   AND  OXIDATION   REACTIONS  53 

so  that  when  the  usual,  amount  of  reducing  agent  is  added  to  the 
charge  to  obtain  a  2O-gram  lead  button,  it  is  found  that,  due  to 
the  oxidizing  power  of  the  ore,  the  button  is  deficient  in  size. 
The  oxidizing  ingredients  of  an  ore  are  generally  hematite  (Fe2O3), 
magnetite  (Fe3O4),  and  manganese  oxides;  i.e.,  MnO2.  The  reac- 
tion which  takes  place  is  as  follows: 

2Fe2O3  +  C  =  4FeO  +  CO2 

One  gram  of  Fe2O3  requires  0.037  gram  °f  carbon  to  reduce 
it  to  FeO. 

In  order  to  determine  the  oxidizing  power  of  an  ore,  make  up 
the  following  charge,  if  the  ore  consists  mostly  of  base.  When 
considerable  silica  is  present  in  the  ore,  decrease  the  silica  in  the 
charge : 

i  assay  ton  of  ore  15  grams  SiO2 

20  grams  Na2CO3  i  •  5  grams  coal 

90  grams  PbO  Borax  glass  cover 


VI 


THE  CRUCIBLE  ASSAY;  ASSAY  SLAGS 

IN  almost  every  instance,  when  a  crucible  assay  is  to  be  made, 
the  ore  and  the  fluxes  added  are  thoroughly  incorporated  by 
mixing,  so  that,  theoretically  at  least,  every  particle  of  the  ore 
is  in  contact  with  a  particle  or  particles  of  fluxes  and  reducing 
agent,  the  most  favorable  condition  to  produce  a  thorough  reac- 
tion among  them.  The  separation  of  the  precious  metals  is 
dependent  upon  their  affinity  for  metallic  lead,  forming  an  alloy 
of  lead,  gold  and  silver,  in  which  lead  greatly  preponderates, 
and  which  readily  settles  by  gravity  from  the  balance  of  the 
ore  and  fluxes  which  have  united  to  form  a  slag.  The  ore  to  be 
assayed  must,  in  all  instances,  be  in  a  finely  crushed  condition, 
varying,  in  American  practice,  from  8o-mesh  up  to  2oo-mesh 
material.  What  takes  place  within  the  crucible  depends  upon 
some  or  all  of  the  following  factors: 

1.  The  fineness  of  crushing.     Are  all  the  particles  of  gold 
and  silver,  or  their  alloy,  present,  entirely  set  free  from  the  in- 
closing gangue?     In  some  ores  this  takes  place  with  much  coarser 
crushing  than  in  others.     In  other  ores  the  values  are  so  finely 
disseminated  that  all  are  not  set  free  within  the  limits  of  crushing 
as  carried  out. 

2.  The  mode  of  occurrence  of  the  gold  and  silver.     Is  it  in 
the  free  state,  as  is  most  generally  the  case  with  gold,  or  are  the 
precious  metals  in  the  form  of  a  more  or  less  complex  mineral 
compound  (tellurides,  argentite,  etc.),  which  must  be  decomposed 
before  the  gold  and  silver  will  alloy  with  the  lead? 

3.  The   physical   properties   of  the   slag  produced;   e.g.,   its 
formation  point,  its  fluidity  at  temperatures  somewhat  above  its 
formation  point,  and  its  fluidity  after  superheating. 

4.  The  chemical  nature  of  the  slag,  its  acidity  or  basicity, 
the  nature  of  the  bases  present,  more  particularly  copper,  zinc, 
antimony,  manganese,  iron,  etc. 

54 


THE  CRUCIBLE  ASSAY;   ASSAY   SLAGS  55 

If  a  crucible  be  broken  open  and  its  contents  examined  shortly 
after  fusion  has  commenced,  these  will  be  found  to  consist  of  a 
heterogeneous  mass  through  which  are  scattered  innumerable 
particles  of  lead,  both  microscopic  and  macroscopic.  The  larger 
particles  have  been  formed  by  the  coalescence  of  the  smaller  par- 
ticles, gradually  settling  through  the  charge  toward  the  bottom 
of  the  crucible  to  form  the  final  lead  button  as  the  temperature 
rises  and  the  charge  becomes  more  fluid  and  less  resistant.  It  is 
evident  that  the  completeness  of  the  collection  of  the  precious 
metals  depends  upon  the  main  factors  already  outlined.  The 
temperature  at  which  charcoal  or  argol,  etc.,  begins  to  react  with 
PbO  to  form  Pb1  is  well  below  906°  C,  the  melting-point  of  PbO. 
The  formation  point  of  a  borate  silicate,  PbO,Na2O,4SiO2,2B2O3 
(Seger  Cone  No.  0.022)  the  constituents  of  which  are  contained 
in  nearly  all  assay  charges,  is  590°  C. 

In  the  fusion  of  a  mixture  containing  silica,  various  bases 
and  borax  glass,  that  silicate-borate  having  the  lowest  formation 
point  will  form,  and  then  as  the  temperature  rises  absorb  either 
silica  or  base  or  both,  as  these  are  in  excess  of  the  ratio  required 
to  form  the  lowest  formation-point  compound.  If  the  temper- 
ature does  not  rise  high  enough  to  cause  this  absorption,  the 
excess  of  silica  or  base  or  both  will  remain  in  suspension  in  the 
formed  silicate-borate,  practically  in  an  unaltered  condition.  If 
the  formed  silicate,  etc.,  constitutes  the  greater  part  of  the  mass, 
there  will  be  an  imperfect  non-homogeneous  slag;  if  the  excess 
of  silica  or  base  forms  the  greater  part  of  the  material,  there  will 
be  a  slightly  fritted  mass. 

Taking  the  simplest  case,  and  also  the  rarest,  that  of  an  ore 
containing  free  gold  completely  liberated  by  crushing,  the  particle 
of  lead,2  formed  at  a  comparatively  low  temperature,  can  unite 
at  once,  as  soon  as  formed,  with  the  gold  particle  not  inclosed 
in  gangue  and  commence  settling  to  the  bottom  to  form  the 
lead  button.  It  is  evident  that  in  this  instance  the  homogeneous 
fusion  and  chemical  decomposition  of  the  ore  are  immaterial. 
Taking,  however,  the  far  more  common  case,  in  which  the  values 

1  CO  reacts  with  PbO  to  form  Pb  at  100°  C.     H  reacts  with  PbO  to  form 
Pb  at  310°  C.  —  Roscoe  and  Shorlemmer,  1892,  Vol.  XI,  part  i,  p.  282. 

2  There  will  probably  be  many  particles  of  lead  for  each  gold  particle  present, 
so  that  no  gold  will  escape  for  lack  of  lead. 


56  A   MANUAL  OF   FIRE  ASSAYING 

are  not  completely  liberated  by  crushing,  it  is  evident  that  the 
particle  of  gold  still  inclosed  within  the  gangue  cannot  be  reached 
by  the  lead  already  reduced,  and  it  becomes  practically  essential 
to  hold  the  lead  in  place  until  the  ore  particle  containing  the  gold 
is  thoroughly  broken  up  chemically  and  liquefied,  so  that  the 
lead  can  absorb  the  gold.  If  the  lead  settles  through  the  charge 
before  this  decomposition  takes  place,  gold  will  remain  in  the 
slag.  The  only  way  to  control  this  condition  is: 

(a)  By  fine  crushing,  liberating  the  values  as  completely  as 
possible. 

(b)  By  the  choice  of  a  slag  having  the  proper  physical  proper- 
ties, i.e.,  a  low  formation  point  and  a  viscous  nature  near  the 
formation  point. 

(c)  By  a  comparatively  slow  fusion  during  the  early  stages 
of  the  assay,  to  prevent  as  much  as  possible  the  rapid  settling 
away  of  the  lead  particles  through  the  still  existing  interstices  of 
the  charge. 

Where  compounds  of  the  precious  metals  are  in  the  ore,  such 
as  argentite  (Ag2S),  tellurides,  calaverite  and  sylvanite  (Au, 
AgTe2),  etc.,  these  are  readily  decomposed  by  the  litharge  as 
follows : 

Ag2S  +  2PbO  =  2PbAg  +  SO2 

The  tellurides  will  be  especially  considered  in  Chapter  X,  on 
"Special  Methods  of  Assay." 

Assay  Slags.  —  An  assay  slag  from  the  crucible  assay  consists 
in  most  instances  of  silicates  and  borates  of  the  metallic  bases. 
While  usually  of  a  homogeneous  nature,  a  slag  is  not  to  be  con- 
sidered a  chemical  compound,  but  rather  as  an  isomorphous 
mixture  of  certain  compounds.  There  may,  perhaps,  be  no 
chemical  compound  in  the  slag;  litharge,  for  example,  will  readily 
unite  with  SiO2  in  varying  proportions,  and  after  fusion  will  be 
homogeneous,  in  all  respects  appearing  similar  to  solutions,  as 
salt  and  water.  Salt  can  be  dissolved  in  water  in  all  proportions 
up  to  23.6  per  cent.,  the  solution  being  homogeneous.  In  the 
same  way  it  may  be  said  that  PbO  can  dissolve  SiO2  within  certain 
limits,  or  vice  versa.  It  is  probable  that  in  this  mixture  certain 
chemical  compounds  exist  which  go  into  solution  in  the  excess 
of  PbO  or  SiO2  present.  Since  these  mixtures  or  solutions  are 


THE  CRUCIBLE  ASSAY;  ASSAY   SLAGS 


57 


fluid  only  at  comparatively  high  temperatures,  and  solid  at 
ordinary  room  temperatures,  the  term  "solid  solutions"  is  used 
for  them.  In  a  similar  way  all  the  common  bases,  FeO,  Na2O, 
K2O,  CaO,  MgO,  A12O3,  ZnO,  MnO,  etc.,  form  silicates  which  are 
relatively  soluble  within  each  other,  so  that  a  slag  may  be  a 
complex  solution  of  various  silicates.  Boric  acid  and  alka- 
line borates  act  in  a  similar  way  to  silica,  and  the  assay  slag  may 
consist,  and  in  most  cases  does  consist,  of  a  solution  of  sili- 
cates and  borates  of  various  bases. 

Silicates  are  defined  in  degree  by  the  ratio  of  oxygen  in  the 
base  to  that  in  the  acid.     The  chemical  classification  is  as  follows: 

TABLE   IV,  — SILICATE   DEGREES 


NAME 

OXYGEN  RATIO,  BASE  TO  ACID 

EXAMPLE 

Orthosilicate 

i  to  i 

MgO  FeO  SiO2 

\Ietasilicate 

I   to  2 

MgO  CaO  2SiO2 

Sesquisilicate  
Bisilicate 

i  to  3 

I   to  4 

K2O.Al2O3.6SiO2 
CaO  2SiO2 

The  metallurgical  classification  is  made  on  the  same  basis, 
i.e.,  oxygen  in  the  base  to  that  in  the  acid,  but  is  somewhat 
different.  It  is  the  one  adopted  in  these  notes. 

TABLE   V.  —  SILICATE   DEGREES 


FORMULA,  RO  (BASE) 

NAME 

FORMULA,  R2O3  (BASE) 

4RO,  SiO2  .  '  
2RO,  Si02  
4RO   sSiOo 

Subsilicate 
Monosilicate 
Sesq  uisilicate 

4R2O3,  3SiO2 
2R2O3,  3SiO2 
4R2O3,  pSiOo 

RO,  SiO2  

Bisilicate 

R2O3,    3SiO; 

2RO,  3SiO2  

Trisilicate 

2R2O3,  9SiO2 

Borates  may  be  classified  in  a  somewhat  similar  manner. 

In  general,  it  may  be  stated  that  the  higher  the  silicate  degree, 
the  more  infusible  is  the  mixture,  and  that  a  polybasic  mixture, 
one  of  many  bases,  is  more  easily  fusible  than  one  of  few.  These 
general  statements  are  not  without  exceptions,  for  certain  bi- 
silicates  and  trisilicates  have  a  lower  fusing  point  than  the  corre- 


58  A  MANUAL  OF   FIRE  ASSAYING 

spending  monosilicate,  etc.  It  also  depends  greatly  upon  the 
base  what  the  fusibility  of  the  silicates  will  be.  PbO,  Na2O,  and 
K2O  give  easily  fusible  silicates;  FeO  and  MnO  give,  comparatively, 
readily  fusible  silicates;  A12O3,  CaO,  and  MgO  give  difficultly 
fusible  silicates.  When,  however,  silicates  of  all  these  various 
bases  are  mixed  and  go  into  solution  as  a  homogeneous  mass, 
the  effect  of  this  mixture  on  the  melting-point  of  the  mass  is 
often  to  lower  it.  In  fact,  the  silicate  mixtures  are  to  be  looked 
upon  from  the  same  point  of  view  as  metallic  alloys;  there  may 
be  eutectic  mixtures,  i.e.,  mixtures  of  two  or  more  constituents 
which  have  a  lower  melting-point  than  either  of  the  constitutents, 
as  is  illustrated  in  the  accompanying  diagram.1 

Rhodonite  Hypersthene 

.2  (MnO)  2  (Si  (V  2  (Fe  O)  2  (Si  O2; 


1100°C 


1000  C 


1100  C 


1050°  C 


1000  C 


Hypersthene 
20 40 60 80 100-/ 

100^  80  60  40  .20  0?£ 

.Rhodonite 

FIG.  37a.  —  FREEZING-POINT  CURVE;  RHODONITE  HYPERSTHENE 

When  approximately  80  per  cent,  of  rhodonite  (the  bisilicate 
of  manganese)  is  fused  with  20  per  cent,  of  hypersthene  (the 
bisilicate  of  iron),  the  melting-point  is  appreciably  lowered. 
CaSiO3  (CaO.SiO2)  and  Na2SiO3  (Na2O.SiO2)  thus  form  a  eutectic 
mixture  of  80  per  cent.  Na2SiO3  and  20  per  cent.  CaSiO3. 

Typical  Assay  Slags.  —  A  slag  of  low  formation  temperature 
and  considerable  viscosity  at  that  temperature  corresponds  to 
Seger  Cone  No.  0.022  —  Na2O.PbO.4SiO2.2B2O3,  590°  C.  This 
may  be  written:  PbO.4SiO2.Na2B4O7  (borax  glass). 

By  calculation  from  the  atomic  weights  the  following  charge 
will  yield  this  slag: 

1  J.  H.  L.  Vogt,  "Die  Silicat-Schmelzlosungen,  II,  Christiana. 


THE   CRUCIBLE   ASSAY;  ASSAY   SLAGS 


59 


PbO  33 . 3  grams 

SiO2    • 36.2  grams 

Na2B4O7 30-4  grams 

The  slag,  corresponding  to  Seger  Cone  0.017  and  melting  at 
740°  C,  may  be  desirable  for  aluminous  ores: 

(Na2O.  PbO.  Al2O3.6SiO2.2B2O3),  which  maybe  written  (Na2B4O7, 
PbO,  A12O3,  6SiO2). 

The  following  charge  will  yield  this  slag: 


Na2B4O7 

PbO  . 


...  .22.9  grams 
....24.9  grams 


A1203 
SiO2 


.  .  1 1 . 5  grams 
.  .40.7  grams 


TABLE   VI.  — ASSAY   SLAGS' 


o3     c3 

SILICATE 

*  JL 

FORMULA 

"rt     C    ''~! 

REMARKS 

DEGREE 

S  U  ^ 

8   2  £ 

o-  3   £ 

CH 

i.  2Na2O.SiO2  

Monosilicate 

IO7O 

Vitreous,           colorless, 

\ 

transparent. 

2.  Na2O.SiOo 

Bisilicate 

IOQO 

Stonv     white     crystal- 

A wyw 

line. 

3.  2PbO.SiO2    

Monosilicate 

I03O 

Vitreous,     It.      yellow, 

transparent. 

4.  PbO.SiO2    

Bisilicate 

10^0 

Vitreous,      It.      yellow, 

J 

transparent. 

5.  Na2O.FeO.SiO2 

M^onosilicate 

1070 

Very  fluid,  stony  black. 

6.  NaoO.FeO.2SiO2    

Bisilicate 

J.  \S  j  W 

1070 

Vitreous,  black. 

7.  PbO.FeO.SiO2  

Monosilicate 

/ 
1  100 

Resinous,  black. 

8.  Na2O.PbO.SiO2  

Monosilicate 

1020 

Vitreous,   yellow-green. 

9.  Na2O.PbO.2SiO2  

Bisilicate 

IO30 

Vitreous,  yellow-green. 

10.  2(PbO.FeO.CaO)3SiO2    .  .  . 

Monosilicate 

IIIO 

Vitreous,  black. 

ii.  Na2O.PbO.FeO.CaO.2SiO, 

Monosilicate 

1030 

Vitreous,  black,  con- 

tains sq.  crystals. 

12.  Na2O.PbO.FeO.Ca0.4Si02 

Bisilicate 

1  100 

Vitreous,  black. 

13.  2(Na2O.PbO.CaO)3SiO2    .  . 

Monosilicate 

1090 

Stony,  light  yellow. 

14.  2(Na2O.FeO.CaO)3SiO2  .  .  . 

Monosilicate 

1150 

Viscous,     stony,    gray- 

brown  . 

15.  2(Na,O.PbO.FeO)3Si02  .  .  . 

Monosilicate 

1030 

Vitreous,  black. 

1  Elmer  E.  West,  Laboratory,  S.  D.  School  of  Mines,  1904. 
Stony  slags  indicate  incomplete  solution  of  some  of  the  ingredients. 


6o 


A  MANUAL  OF   FIRE  ASSAYING 


A  partial  replacement  of  the  silica  by  borax  glass  in  the 
foregoing  slags  will  appreciably  lower  the  formation  points. 

Bases  such  as  FeO,  CaO,  MgO,  MnO,  BaO,  and  A12O3  are 
present  in  greater  or  lesser  quantity  in  almost  all  ores,  and  SiO2 
is  present  in  practically  every  ore,  so  that  such  slags  as  those 
outlined  must  necessarily  be  made.  The  easily  fusible  bases 
PbO  and  Na2O  serve  to  lower  the  formation  point  of  the  slag. 
If  it  is  accepted  that  the  composition  of  the  slag  in  the  assay  is 
practically  the  constant  factor,  it  is  evident  that  when  the  ap- 
proximate composition  of  the  ore  is  known,  we  will  add  either 
basic  or  acid  fluxes,  in  such  proportions  as  to  produce  the  proper 
slag  decided  upon.  The  most  desirable  constitution  for  an  assay 
slag  in  general  is  that  of  a  monosilicate  or  a  sesquisilicate,  some- 
times, but  more  rarely,  a  bisilicate.  If  the  ore  is  basic  a  bisilicate 
may  be  approached,  if  acid  a  monosilicate,  or  even  a  sub-silicate, 
in  order  to  insure  complete  decomposition. 

The  accompanying  table  will  simplify  slag  calculations: 

TABLE  VII.  — THE  CALCULATION  OF  SLAGS  ' 


UNIT  MOLECULAR  BASE  RATIO;  E.G.,  PfiO:  NA2O:  FEO,  ETC.  =  i:  i:  i 


ONE  PART  OF 
BASE  BY  WEIGHT 

PARTS  OF  OTHER  BASES  NECESSARY 

PARTS    OF    SiO2 
NECESSARY    FOR 
MONOSILICATE 

Na2O 

PbO 

CaO 

A1203 

FeO 

ZnO 

Na2O 

I    OOO 

"?    ^QO 

O    QO"? 

i  646 

i   1  60 

I     "?!  I 

o  486 

PbO    

0.279 

I.  OOO 

O.252 

Q-459 

0-323 

0-365 

o.  136 

FeO  

0.862 

3-095 

0.779 

1.419 

I  .000 

I.I30 

0.419 

CaO    

1.108 

3-976 

I  .000 

1.823 

1.284 

1.452 

o-539 

A12O3  

0.608 

2.181 

0.549 

i  .000 

0.705 

o-797 

0.886 

CuO    

0.780 

2.801 

0.704 

1.284 

0.905 

1.023 

o-379 

ZnO 

0.763 

2.738 

0.689 

i-255 

0.885 

i  .000 

0.371 

One  Part  by 

Weight  of  SiO2 

Na2O 

PbO 

CaO 

A12O3 

FeO 

ZnO 

CuO 

requires  to  form 

2.07 

7-36 

1.86 

1.14 

2  .40 

2.70 

2.63 

the  Monosilicate 

parts 

parts 

parts 

parts 

parts 

parts 

parts 

1  Based  on  Balling's  table. 


THE  CRUCIBLE   ASSAY;   ASSAY   SLAGS  61 

When  a  bisilicate  is  to  be  calculated,  the  silica  required  for 
a  monosilicate  is  determined  and  then  multiplied  by  two.  Vice 
versa,  when  the  bases  for  the  monosilicate  have  been  calculated 
and  a  bisilicate  is  to  be  formed,  the  bases  must  be  divided  by 
two.  The  same  reasoning  applies  to  other  silicate  degrees. 

Example  of  the  Calculation  of  an  Assay  Slag.  —  The  problem  is 
to  calculate  a  charge  to  produce  the  following  monosilicate: 
Na2O.PbO.FeO.CaO.2SiO2.  Taking  as  the  unit  10  grams  of  Na2O, 
it  follows  from  the  preceding  table  that  the  weights  of  the  sub- 
stances required  are: 

Na2O    10  X  i  =10.0    grams 

PbO 10  X  3.59     =35-9    grams 

FeO    10  Xi.i6    =11.6    grams 

CaO 10  X  0.903  =    9.03  grams 

The  silica  required  will  be: 

for  the         Na2O    10    X  0.486  =    4.86  grams 

PbO   35  . 9    X  o .  136  =    4 . 86  grams 

FeO    n. 6    Xo. 419  =    4 . 86  grams 

CaO  9 . 03  X  o .  539  =    4 . 86  grams 

Total    19-44  grams  SiO2 

The  silica  may  be  determined  by  calculating  it  for  one  base 
and  multiplying  that  figure  by  the  number  of  oxygen  molecules 
in  the  bases  present,  after  having  reduced  the  slag  formula  to  its 
lowest  possible  terms.  Before  making  up  the  charge,  it  is  essen- 
tial to  remember  that  the  Na2O  in  this  instance  is  furnished  in 
the  form  of  NaHCO3,  which  contains  approximately  40  per  cent, 
of  Na2O,  and  that  the  FeO  is  furnished  by  an  iron  ore  of  the 
following  approximate  composition: 

Fe2O3,  80  per  cent.;  SiO2,  17  per  cent. 

The  lime  is  furnished  by  limestone,  CaCO3,  practically  pure. 

It  is  also  necessary  to  provide  a  lead  button;  so  extra  litharge 
must  be  furnished.  To  reduce  the  lead,  coal  dust  is  added. 
Some  of  the  coal  will  be  used  up  to  reduce  the  Fe2O3  to  FeO. 
Hence  the  following  calculations  are  to  be  made:  10  grams  Na2O 

are  required;  therefore  —  X  100  =  25  grams  of  NaHCO3  must 

4° 
be  added.     PbO  contains  92  per  cent,  of  Pb;  therefore,  in  order 


62  A  MANUAL  OF   FIRE  ASSAYING 

20  X  i oo 
to  obtain  a  2O-gram  lead  button,  -  -  =  22  grams  of  PbO 

must  be  added,  in  addition  to  the  35.9  grams  for  the  silicate  - 
a  total  of  57.9  grams  of  PbO.     Eleven  and  six-tenths  grams  of  FeO 
are  required.      Fe2O3  consists  of  90  per  cent,  of  FeO  and  10  per 

cent,  of  O2;  and  as  the  ore  is  80  per  cent,  of  Fe2O3,  -    '• ~ 

=  1 6.  i  grams  of  ore  will  be  required.     The  limestone  contains  54 
per  cent.  CaO;  therefore,  -  =  16.7  grams  of  limestone 

will  be  required. 

The  coal  in  use  has  a  reducing  power  of  20  grams  of  lead  per 
gram  of  coal. 

The  following  reaction  takes  place  between  carbon  and  the 
Fe20, 

2Fe2O3  +  C  =  4FeO  +  CO2. 

One  gram  of  Fe2O3  requires  -       =  0.037  grarn  of  charcoal. 

,  .        ,    20  X  100 
But  as  the  coal  used  is  only  -  -  =  58  per  cent,  as  strong  as 

34-4 
charcoal,  the  following  quantity  will  have  to  be  added  to  the 

1 6. i  grams  of  Fe2O3  to  reduce  it: 

0.037  X  16.  i  X  80 

g—       -  =  0.82  gram  coal 

To  this  must  be  added  ,i  gram  for  the  reduction  of  the  2o-gram 
lead  button,  giving  1.82  grams  of  coal  to  be  added. 

Since  the  iron  ore  contains  silica,  this  is  to  be  deducted  from 
the  silica  calculated.  The  amount  of  SiO2  in  the  ore  is  16.1  X  1*7 
per  cent.  =  2.74  grams. 

The  correct  charge  then  is: 

25        grams NaHCO3  16. 7    grams limestone 

57.9    grams PbO  16.7    grams silica (19. 44  —  2.  74) 

16.  i     grams FeoOs  (iron  ore)         i  .82  grams coal 

Salt  cover 

Following  is  the  calculation  of  the  same  slag,  but  for  a  quartz 
ore  containing  95  per  cent.  SiO2.  The  formula  for  the  slag  is: 
Na2O.PbO.FeO.CaO.2SiO2.  Taking  as  the  unit  i  assay  ton  of 
ore,  or,  in  round  numbers,  30  grams,  this  will  contain  28.50  grams 


THE  CRUCIBLE  ASSAY;   ASSAY    SLAGS  63 

of  SiO2.  These  28.5  grams  are  to  be  divided  into  4  equal  parts 
to  satisfy  the  4  bases  present.  Therefore,  7.1  grams  of  SiO2 
will  go  to  such  an  amount  of  each  base  as  will  form  a  monosilicate. 

7.1  grams  SiC>2  require  7.1  X  2.07  =  14.7     grams  Na^O 
7.1  grams  SiC>2  require  7.1  X  7-36  =  52.25  grams  PbO 
7.1  grams  SiO2  require  7.1  X  2.40  =  17.04  grams  FeO 
7.1  grams  SiC>2  require  7.1  X  1.86  =  13.20  grams  CaO 

,   .      1 4 .  7O  X   I OO 

The  bicarbonate  of  soda  required  is  -  -  =  37  grams. 

4° 
The  PbO  required  is  52.25  +  22  =  74.25  grams  for  the  lead 

button. 

TU      r-  rr\     f  -j      •*.    \  •      J   •      1 7- 04  X    IOO 

The  FeCO3  (sidente)  required  is-   — ^ —     -=  27  grams. 

TM          ,.  '         4     '        13.20    X     IOO 

The  limestone  required  is  -  —  =  24.4  grams. 

54 
The  complete  charge  is: 

i       assay  ton  of  ore  27       grams FeCOs 

37      grams NaHCOs         24.5  grams CaCOs 

74      grams PbO  i       gram coal 

Salt  cover 

In  one  case  the  ore  is  of  a  basic  nature  —  hematite  and  lime- 
stone (17  grams  of  each),  and  in  the  other  case  it  is  of  an  acid 
nature  —  quartz;  yet  the  slag  produced  is  the  same  in  both 
cases.  This  brings  out  the  fact  that  the  slag  is  the  constant  and 
that  fluxes  are  added  of  such  nature  and  in  such  quantity,  deter- 
mined by  the  ore,  as  to  produce  a  slag  of  fairly  constant  composi- 
tion. It  is  to  be  noted  that  the  slag  made  in  the  two  assays 
contains  four  bases,  PbO,  Na2O,  FeO,  CaO,  and  that  these  are 
present  in  unit  molecular  base  ratio.  As  a  matter  of  fact,  the 
assayer  rarely  adds  CaO  or  FeO  as  fluxes,  but  when  these  are 
present  in  the  slag,  they  are  derived  from  the  ore.  The  bases 
added  as  fluxes  are  practically  limited  to  three,  PbO,  Na2O  and, 
at  times,  K2O,  so  that  when  an  ore  consisting  chiefly  of  SiO2  is 
to  be  assayed,  the  slag  made  will  approximate  a  monosilicate  and 
borate  of  lead  oxide  and  soda. 

The  table  of  assay  slags  given  mentions  only  those  in  which 
the  bases  are  present  in  the  unit  molecular  ratio.  It  is  evident 
that  where  an  ore  is  considered  in  which  numerous  bases  are 
present,  these  are  not  contained  in  the  unit  molecular  ratio,  so 


64 


A   MANUAL  OF   FIRE  ASSAYING 


that  the  formula  of  the  slag  made  will  rather  have  this  general 
form: 

(xPbO,  yNa2O,  zFeO,  tMgO)  vSiO2, 

in  which,  for  a  monosilicate,  considering  the  letters  as  oxygen 
coefficients,  x  +  y  +  z  +  t  =  2v.  In  order  to  get  a  slag  of  low 
formation  point,  the  coefficients  of  the  more  infusible  bases,  such 
as  CaO,  MgO,  A12O3,  will  have  to  be  materially  smaller  than  those 
of  the  more  fusible  bases,  PbO,  Na2O,  and  FeO. 

In  assay  practice,  it  is  neither  possible  nor  desirable  to  make 
analyses  of  ore  before  assaying  for  gold  and  silver  The  assayer, 
however,  is  supposed  to  have  a  good  working  knowledge  of 
lithology  and  mineralogy,  which  will  enable  him  to  form  a  correct 
judgment  of  the  contents  of  his  ore  within  fair  limits.  It  will  be 
comparatively  easy  for  him  to  tell  at  once  whether  he  has  lime- 
stone or  dolomite,  or  an  ore  containing  much  limonite  or  hema- 
tite or  the  iron  sulphides;  or  whether  magnesia,  barium  or  other 
minerals  are  present,  and  in  what  general  proportions.  Following 
are  analyses  of  silicious  and  lead  antimonial  ores: 

TABLE   VIII.  —  SILICIOUS   ORES 


No.  i 

No.  2 

No.  3 

No.  4 

No.  5 

No.  6 

Gold  

o  .  63  oz. 

0.85  oz. 

3-35oz. 

2.00  OZ. 

0.78  oz. 

0.90  oz. 

Silver   

2.00  OZ. 

6.08  oz. 

1.75  oz. 

0.62  oz. 

I  .00  OZ. 



per  cent. 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

Silica    

65-38 

80.00 

80.90 

84.80 

77-38 

93-72 

Iron 

1  3  4.0 

7  •  ^O 

0  -04 

7-  ^o 

•7  .  CA 

2.67 

Sulphur    

ii  .40 

4.40 

4-  ^3 

o.  75 

4.42 

o.  60 

Arsenic 

o  oo 

2  .OO 

o.  20 

o.oo 

o.  55 

O.O2 

Antimony  ...... 

trace 

trace 

trace 

trace 

trace 

0.089 

7inr 

Manganese 

trace 

0-54 

trace 

0.96 



0.082 

Alumina  

c  .42 

i  .  79 

i  .  70 

I  .02 

2.80 

3-53 

T    1TV10 

.Lime  

1.70 

o.  50 

Magnesia  

o.  20 



trace 



trace 



THE   CRUCIBLE   ASSAY;   ASSAY    SLAGS 
TABLE   IX.  — LEAD    ANTIMONIAL   ORES 


No.  i 

No.  2 

No.  3 

Silica  
Ferrous  oxide  
Alumina    

60.  i     per  cent. 
5.2     per  cent. 
9.5     per  cent. 

57.65  per  cent. 
0.70  per  cent. 
1.40  per  cent. 

59.50  per  cent. 
4.60  per  cent. 
9.00  per  cent 

Magnesia    
Lime                

2.68  per  cent, 
trace 

2.09  per  cent, 
trace 

3.00  per  cent, 
trace 

Lead 

10.6    per  cent. 

1  6.  86  per  cent 

10  i     per  cent 

Antimony  
Sulphur  

4.4    per  cent. 
0.5     per  cent. 

11.84  per  cent. 

7.55  per  cent. 
0.44  per  cent 

Wa  tpr 

TABLE   X.  — HEMATITE 


TABLE   XL  — LIMESTONE 


ANALYSIS  *OF  A  HEMATITE 

ANALYSIS  OF  A  LIMESTONE 

Silica 

14.20    per  cent. 
73.68  per  cent. 
5.03    per  cent. 
0.57    per  cent. 
0.19    per  cent, 
o.ioi  per  cent. 

Silica    
Alumina  and  ferric  oxide 
Magnesia  
Lime 

1.94  per  cent. 
0.68  per  cent. 
0.18  per  cent. 
53.61  per  cent. 
43.81  per  cent, 
o.i  i  per  cent. 

Ferrous  oxide  
Alumina  
Lime 

Manganous  oxide  . 
Phosphorus   

Carbonic  acid  
Water 

These  analyses  are  given  to  show  what  the  chief  base  constit- 
uents may  be,  and  how  ores  will  range  from  acid  types  to  basic 
ones.  Whenever  sulphides  are  present,  it  is  to  be  noted  that  the 
oxidation  of  these  leaves  basic  oxides  to  be  fluxed. 

At  times,  instead  of  silicate  and  borate  slags,  it  is  desirable  to 
make  oxide  slags  in  the  crucible  assay.  This,  of  course,  can  only 
be  done  when  silica  is  absent  from  the  ores,  or  when  a  very  large 
excess  of  litharge  is  used  in  the  fusion.  Litharge,  which  melts 
at  906°  C,  possesses  the  property  of  dissolving  or  holding  in 
suspension  certain  quantities  of  other  metallic  oxides.  These 
slags  are  discussed  in  the  chapter  "Assay  of  Impure  Ores." 

The  charge  for  the  monosilicate  of  lead  and  soda  is  (using  the 
unit  molecular  base  ratio) : 

0.5  assay  ton  silica  or  quartz  ore 
39      grams  NaHCO3 
55       grams  PbO 
Borax  glass  cover 


66  A  ,MANUAL  OF   FIRE  ASSAYING 

For  the  bisilicate  it  is: 

o .  5  assay  ton  silica  or  quartz  ore 
20      grams  NaHCOs 
28      grams  PbO 
Borax  glass  cover 

Allowing  for  a  2o-gram  lead  button,  the  charges  are: 


No.  i 
MONOSILICATE 
ORE  (QUARTZ), 

0.5  ASSAY  TON 

No.  2 
SESQUISILICATE 
(APPROXIMATE) 
ORE  (QUARTZ), 

*i       0.5   ASSAY  TON 

No.  3 
BISILICATE 
ORE  (QUARTZ), 
0.5  ASSAY  TON  , 

NaHCOs   ...     39  grams 

NaHCOa               30  grams 

NaHCOs             20  grams 

PbO  77  grams 

PbO   60  grams 

PbO                      ^o  grams 

Coal  i  gram 

Coal  .    .                    i  gram 

Coal                      i  gram 

Borax  glass  cover 

Borax  glass  cover 

Borax  glass  cover 

All  of  the  above  charges  will  yield  satisfactory  slags  in  an 
ore  assay  if  the  ore  is  of  the  nature  described.  No.  3  is  the 
cheapest  in  point  of  cost;  No.  2  is  the  one  most  frequently  made. 

Color  of  Slags.  —  Most  slags  from  ore  assays  will  be  from 
light  to  very  dark  green  in  color  or  almost  black,  this  color  being 
due  to  various  proportions  of  ferrous  silicate.  When  iron  is 
absent,  the  color  of  lead  silicates  (yellow)  may  predominate,  or 
white  and  gray  or  colorless  slags,  due  to  silicates  of  CaO.MgO.ZnO, 
etc.,  be  produced.  Copper  produces  red  slags,  due  to  cuprous 
silicate.  Cobalt  gives  blue  slags.  When  much  lime  is  present 
in  an  ore,  this  is  best  calculated  to  a  bisilicate  or  even  higher,, 
while  the  other  bases  can  be  calculated  to  the  monosilicate. 


VII 


CUPELLATION 

CUPELLATION  has  for  its  object  the  oxidation  of  the  lead  in 
the  gold,  silver,  etc.,  alloy  to  PbO,  which  in  part  (95  per  cent.) 
is  absorbed  by  the  cupel,  and  in  part  (5  per  cent.)  volatilized. 
The  silver  and  gold  of  the  alloy  are  left  as  a  metallic  bead.  The 
process  is  carried  out  in  cupels.  Cupels  are  shallow  porous  dishes, 
made  generally  of  bone-ash.  Leached  wood-ashes  (particularly 
from  beech-wood)  and  lime  and  magnesia  have  also  been  used 
for  cupels.  A  mixture  of  bone-ash  and  leached  wood-ashes,  in 
the  proportion  of  i  to  2  and  2  to  i  respectively,  has  been  used, 
and  is  said  to  give  a  much  smaller  absorption  of  the  precious 
metals  than  bone-ash  cupels.1 

The  bone  which  yields  the  bone-ash  on  calcining  has  the 
following  composition.2 


SHEEP  BONES 

CATTLE  BONES 

Ca3(PO4)2    

62.70  per  cent. 

58.30  per  cent. 

CaCO3  

7.00  per  cent. 

7.00  per  cent. 

Mg3(PO4)o                                      

1.59  per  cent. 

2.09  per  cent. 

CaF2 

2.17  per  cent. 

1.96  per  cent. 

Organic  matter  

26.54  per  cent. 

30.58  per  cent. 

These  bones  will  produce  bone-ash  of  the  following  composition 


No.  i 

No.  2 

Ca3(PO4)2 

84..  T.Q  per  cent. 

83.07  per  cent. 

CaCO3 

9  42  per  cent. 

10.00  per  cent. 

CaF2 

405  per  cent 

3.88  per  cent. 

Mg2(P(V)3  . 

2.  is  per  cent. 

2.08  per  cent. 

1  Kerl,  "Probir  Kunst,"  1886,  p.  91. 
2Hemtz,  "Erdman's  Jour,  of  P.  Chem.,"  Bd. 
67 


S,  S.  24. 


68  A  MANUAL  OF   FIRE  ASSAYING 

The  bone-ash  used  for  cupels  must  be  specially  treated  by 
washing  with  an  aqueous  solution  of  ammonium  chloride  (this 
salt  to  the  extent  of  2  per  cent,  of  the  weight  of  the  bone-ash  to 
be  treated).1  This  reacts  with  CaCO3  and  any  CaO  present,  con- 
verting them  into  CaCl2,  which  is  removed  by  washing  with  water. 
The  presence  of  CaCO3  is  very  undesirable  in  bone-ash  for  cupels, 
as  it  begins  to  give  off  CO2at  700°  C,  about  the  proper  temperature 
for  cupellation,  causing  a  serious  spitting  of  the  lead  button, 
which  entails  a  loss  of  the  precious  metals.  Cupels  should  not 
be  kept  where  the  nitrous  fumes  from  parting  can  be  absorbed 
by  them,  as  these  will  form  Ca(NO3)2  with  any  CaO  that  may 
be  present,  which  also  is  decomposed  about  the  temperature  of 
cupellation.  Bone-ash  melts  at  about  1450°  C.  (Hempel). 

The  physical  nature  of  the  cupel,  especially  as  regards  porosity, 
is  very  important.  For  this  reason  there  should  be  a  careful 
adjustment  of  the  relative  amounts  of  different  sized  particles 
present.  Practically,  only  the  fraction  of  i  per  cent,  of  the 
bone-ash  should  remain  on  a  4O-mesh  screen.  If  there  is  an 
insufficiency  of  fine  particles  in  the  bone-ash,  the  cupel  will  be 
too  porous  and  cause  a  relatively  heavy  absorption  of  gold  and 
silver.  If  the  bone-ash  is  too  fine,  the  cupels  made  from  it  will 
be  too  dense,  prolonging  the  cupellation  and  causing  losses,  mainly 
by  increased  volatilization. 

The  following  is  a  screen  analysis  of  the  bone-ash  commonly 
purchased,  but  which  is  rather  coarse: 

Through  a     20-mesh  screen,  100        per  cent. 

On  a               30-mesh  screen  2.00  per  cent. 

On  a               40-mesh  screen,  6.40  per  cent. 

On  a              6o-mesh  screen,  10.04  per  cent. 

On  a              8o-mesh  screen,  2.00  per  cent. 

On  a            loo-mesh  screen,  11.20  per  cent. 

Through  a  loo-mesh  screen,  68.88  per  cent. 

Cupels  should  be  as  uniform  as  possible  as  regards  density, 
and  for  this  reason  are  best  made  by  machine,  in  which  a  constant 
pressure  may  be  obtained,  rather  than  by  hand  molds.  Fig.  38 
shows  a  good  type  of  cupel  machine.  Considerable  pressure  may 
be  used,  and  the  cupels  made  quite  firm,  j  It  is  not  possible  to 
specify  the  proper  condition  in  definite  terms,  but  a  batch  of 

1  W.  Bettel,  "Jour.  Chem.  and  Met.  Soc.  of  S.  A.,"  Vol.  II,  p.  599. 


CUPELLATION 


cupels,  after  being  made  up  and  carefully  dried  for  at  least  three 
weeks  or  a  month,  should  be  tested  by  cupeling  a  weighed  quan- 
tity (200  mgs.)  of  c.  p.  silver  with  20  grams  of  lead  at  the  proper 
temperature,  700°  C.,  and  the  loss  noted.  It  should  not  exceed 
from  1.5  to  1.8  per  cent. 


FIG.  38.  —  CUPEL  MACHINE 

The  bone-ash  to  be  made  into  cupels  is  mixed  with  from  8  to 
12  per  cent,  of  water,  in  which  is  dissolved  a  little  K2CO3,  or  to 
which  has  been  added  a  little  molasses  or  stale  beer.  After 
making,  the  cupels  should  be  carefully  and  slowly  dried.  If 
possible,  cupels  should  be  several  months  old  before  using.  In 
the  Royal  British  Mint  no  cupels  less  than  two  years  old  are 
used  for  bullion  assays. 

If  cupels  are  too  rapidly  dried,  or  have  been  made  up  too  wet, 
they  crack  and  check  when  placed  in  the  furnace  and  make  the 
assays  conducted  in  them  unreliable. 

The  importance  of  good  cupels  cannot  be  overestimated. 
Very  frequently,  inaccuracies  in  the  assays  are  due  chiefly  to  the 
cupel.  The  shape  of  the  cupel  has  some  influence  on  the  loss  of 


70  A   MANUAL  OF   FIRE   ASSAYING 

precious  metals  by  absorption.  If  the  cupel  is  very  flat  and 
shallow,  so  that  the  molten  lead  covers  a  large  area  and  has 
little  depth,  the  time  of  cupellation  is  decreased  as  the  surface 
exposed  to  oxidation  is  increased,  but  as  the  absorption  of  precious 
metals  is  probably  a  function  of  the  area  exposed,  it  will  be  large 
in  shallow  cupels.1 

Cupellation.  —  When  ready  to  cupel  lead  buttons,  the  cupels 
are  placed,  empty,  in  the  red-hot  muffle  and  allowed  to  remain 
there  for  about  10  minutes  in  order  to  expel  any  moisture,  or 
organic  matter  present  (if  molasses  water  has  been  used  in  making 
them  up).  If  the  buttons  were  placed  into  the  cold  cupel,  the 
lead  would  melt  before  all  the  remaining  moisture  is  expelled, 
which  would  then  pass  up  violently  through  the  molten  lead, 
causing  what  is  termed  "spitting/'  i.e.,  the  projection  of  small 
lead  particles,  carrying  values  from  the  cupel.  Some  cupels, 
made  from  bone-ash  containing  CaCO3,  will  commence  to  spit 
after  the  cupellation  has  proceeded  for  some  time  and  the  tem- 
perature has  risen  to  above  700°  C.  This  can  be  stopped  by 
pulling  the  cupel  to  the  cooler  (front)  part  of  the  muffle,  although 
the  cupellation,  after  spitting,  is  to  be  considered  unreliable 
When  a  piece  of  wood  or  coal  is  placed  in  the  muffle  to  "open  up" 
lead  buttons,  the  cupels  absorb  gases  at  times,  which  later  on, 
when  the  temperature  rises,  are  again  expelled,  with  a  spitting 
of  the  lead. 

When  the  lead  button  is  put  into  the  hot  cupel,  the  lead 
melts  (326°  C.)  and  is  covered  by  a  gray-black  scum.  If  the  lead 
button  is  practically  pure,  as  it  should  be,  this  black  scum  dis- 
appears when  the  lead  reaches  a  temperature  of  675°  C.2  This 
is  called  the  "opening  up"  or  "uncovering"  of  the  lead  button. 
The  molten  lead  then  appears  bright,  begins  to  "drive,"  and 
active  and  rapid  oxidation  commences.  Lead  buttons  should 
uncover  as  soon  as  possible  in  the  muffle.  If  other  and  more 
difficultly  fusible  metals,  such  as  Cu,  Fe,  etc.,  are  present,  the 
temperature  of  uncovering  is  higher  and  the  temperature  required 
for  cupellation  is  higher.  These  foreign  metals  should,  however, 
as  a  general  rule,  be  absent. 

Little  flakes  of  PbO  form  on  the  surface  of  the  molten  lead 

JH.  K.  Edmands,  in  "Eng.  and  Min.  Journ.,"  LXXX,  245. 

2T.  K.  Rose,  in  "Journ.  Chem.  Met.  and  Min.  Soc.  of  S.  A.,'-'  Jan.,  1905. 


CUPELLATION  71 

and  slide  down  the  convex  surface  of  the  button,  and  are  absorbed 
by  the  porous  mass  of  the  cupel.  If  the  temperature  of  the 
cupellation  is  between  700°  and  750°  C,  as  it  should  be,  this 
litharge  is  solid,  as  the  litharge  melts  at  906°  C.  The  temperature 
of  the  lead  itself  during  cupellation  is  probably  higher  than  that 
indicated  by  the  pyrometer  couple  near  it,  owing  to  the  rapid 
oxidation  of  the  Pb.  This  is  indicated  by  the  brighter  color  of 
the  lead. 

Any  foreign  metals,  as  Cu,  Sb,  Fe,  Zn,  etc.,  which  are  present 
are  oxidized  (some  by  the  PbO  formed),  and  absorbed  by  the 
cupel,  if  not  present  in  too  large  amounts. 

Zn  +  PbO  =  Pb  +  ZnO. 

Such  elements  as  Sb,  As,  and  Zn,  when  present  in  the  button, 
are  in  part  volatilized  as  oxides,  and  in  part  absorbed.  When 
cupellation  for  silver  is  carried  on,  the  temperature  should  not 
be  above  700°  C.,  in  which  case  crystals  of  litharge  (feathers) 
form  on  the  side  of  the  cupel  toward  the  muffle  mouth.  If  the 
temperature  is  getting  too  low  for  the  cupel  to  successfully  absorb 
practically  all  of  the  PbO,  these  feathers  form  low  down  in  the 
cupel.  When  the  temperature  is  about  right,  they  form  near 
the  upper  rim  of  the  cupel.  It  is,  however,  to  be  noted  that  the 
draft  through  the  muffle  influences  the  formation  of  feather 
litharge;  i.e.,  if  the  draft  is  strong,  feathers  will  form,  although 
the  temperature  is  somewhat  above  700°  C.  During  cupellation, 
the  door  of  the  muffle  should  never  be  left  wide  open,  but  should 
be  set  slightly  ajar,  so  that  the  cold  air  will  not  strike  directly 
upon  the  cupels.  When  silver  and  gold  are  cupeled  for,  owing 
to  the  higher  melting-point  of  the  silver-gold  alloy,  the  finishing 
temperature  will  have  to  be  750°  C.  at  least. 

As  the  cupellation  proceeds,  the  percentage  of  lead  in  the 
alloy  decreases  and  that  of  Ag  and  Au  increases.  The  litharge 
thrown  off  from  the  center  of  the  button  is  in  larger  specks,  and 
brilliant,  and  the  button  assumes  a  more  rounded  form.  When 
this  phenomenon  appears,  the  button  should  be  pushed  back  into 
the  hotter  part  of  the  furnace  or  the  temperature  of  the  furnace 
raised  somewhat.  When  the  last  of  the  Pb  goes  off,  large  buttons 
are  covered  with  a  brilliant  film  of  colors  (interference  colors) 
and  the  button  appears  to  revolve  axially.  The  colors  then 


72  A   MANUAL  OF   FIRE   ASSAYING 

disappear,  the  bead  becomes  dull,  and  then  again  takes  on  a 
silvery  tinge. 

If  now  the  temperature  of  the  muffle  is  below  that  of  the 
melting-point  of  silver  (962°  C),  or  below  that  of  the  gold-silver 
alloy  constituting  the  bead,  or  if  the  cupel  be  withdrawn  from 
the  furnace,  the  "blick"  or  "brightening"  or  "flash"  of  the 
bead  takes  place;  i.e.,  the  bead  suddenly  becomes  very  bright, 
at  the  moment  of  solidification,  owing  to  the  release  of  the  latent 
heat  of  fusion,  which  raises  the  temperature  of  the  bead  very 
much  for  a  short  time.  The  bead  has  been  in  a  state  of  surfusion, 
i.e.,  in  a  state  of  fusion  below  its  true  freezing-point,  toward 
the  last  of  the  cupeling  operation ;  and  if  it  be  lightly  jarred  or  the 
temperature  allowed  to  drop  still  lower  (by  taking  it  out  of  the 
muffle),  it  suddenly  congeals  and  assumes  a  state  normal  (solid) 
to  the  temperature  existing.1  The  release  of  the  latent  heat, 
raising  the  temperature  of  the  bead,  causes  the  brightening., 
The  "brightening"  of  very  small  beads  is  rarely  noticeable^ 
Silver  and  gold  beads  still  containing  small  amounts  of  Pb  or  Cu 
do  not  brighten  so  noticeably.  If  even  minute  quantities  of  rho- 
dium, iridium,  ruthenium,  osmium,  or  osmium-iridium,are  present, 
buttons  will  not  flash.  Platinum  and  palladium  are  excepted. 

Silver  beads  after  cupellation,  and  at  the  moment  of  solidi- 
fication, also  "sprout."  Just  before  the  silver  bead  becomes 
solid  it  absorbs  oxygen  from  the  air,  the  maximum  absorption 
being  about  22  volumes.  This  oxygen  is  suddenly  expelled  when 
the  bead  solidifies,  causing  a  cauliflower-like  growth  on  the  bead. 
Small  particles  of  silver  may  even  be  projected  from  it  and  cause  a 
serious  loss.  When  gold  is  present  in  the  silver  bead  to  the  extent 
of  33  per  cent,  or  more,  sprouting  does  not  take  place.  Silver 
beads  containing  small  quantities  of  Pb,  Cu,  Zn,  Bi,  etc.,  will  not 
sprout,  so  that  if  a  button  does  sprout  it  is  a  sign  of  purity. 

Buttons  below  5  mgs.  in  weight  do  not  sprout  readily;  large 
buttons,  however,  do.  Sprouting  can  be  prevented  by  slow 
cooling  in  the  muffle,  or  by  having  ready  a  hot  cupel  which  can 
be  set,  inverted,  over  the  one  holding  the  bead,  and  withdrawing 
both  from  the  muffle,  thus  cooling  the  bead  slowly.  Sprouted 
beads  are  to  be  rejected  as  an  assay. 

When   cupeling  for  silver  alone,  or  for  silver  and  gold,  it  is 

1  Rose,  "Metallurgy  of  Gold." 


CUPELLATION 


73 


necessary  to  watch  the  end  of  the  cupellation  carefully,  and  to 
promptly  remove  the  cupel  about  30  seconds  to  i  minute  after 
the  bead  has  become  dull.  A  heavy  loss  of  silver  commences  if 
the  silver  buttons  are  kept  beyond  that  time  in  the  furnace.  If 
silver  is  not  to  be  determined,  but  gold  only,  the  buttons  may 
be  left  in  for  5  to  10  minutes  without  loss  of  gold.  Gold  beads 
will  retain  minute  amounts  of  lead  which  cannot  be  removed  by 
permitting  the  bead  to  stay  in  the  muffle. 

The  bead,  when  cold,  is  taken  from  the  cupel  with  a  pair  of 
pliers,  and  cleaned  of  bone-ash  by  flattening  somewhat  with  a 
hammer.  It  should  be  examined  with  a  glass  to  make  sure  that 
no  bone-ash  adheres  to  it. 

The  bead  should  t>e  either  white  or  yellow,  depending  on  the 
amount  of  gold  present,  round  and  not  flat  (the  latter  indicating 
the  presence  of  foreign  metals),  and  should  possess  a  crystalline 
surface  where  it  adhered  to  the  bone-ash.  It  should  be  firmly 
attached  to  the  bone-ash  of  the  cupel.  If  it  is  not,  this  fact 
indicates  that  lead  is  still  present.  It  should  also  have  no  rootlets 
extending  into  the  cupel.  The  cupel,  after  cupellation,  should 
be  smooth  and  firm,  not  fissured  and  cracked,  and  of  a  light 
yellow  color  when  cold.  Other  colors  indicate  the  presence  of  , 
foreign  metals. 

The  freezing-point  curve  of  lead-silver  (Fig.  39)  will  give 
some  idea  of  the  proper  temperature  of  cupellation.  A  lead 
button  is  to  be  considered  as  an  alloy  of  lead  and  silver  (or  gold) 
which  in  the  process  of  cupellation  undergoes  the  change  from 
practically  pure  lead  to  that  of  pure  silver  (or  gold). 

A  2o-gram  button  containing  200  mgs.  of  silver  contains') 
i  per  cent,  of  Ag.  An  alloy  of  lead  and  silver  containing  4  per 
cent,  of  Ag  is  of  "eutectic  composition"  and  melts  at  303°  C, 
the  melting-point  of  pure  lead  being  327°  C.  Most  assay  buttons 
will  contain  very  much  less  than  i  per  cent,  of  silver  and  will 
melt  practically  at  the  melting-point  of  lead.  Leaving  out  of 
consideration  for  the  moment  that  lead  "uncovers"  at  675°  C. 
in  an  oxidizing  atmosphere,  and  the  proper  temperature  required 
to  cause  a  ready  absorption  of  PbO  by  the  cupel,  it  is  evident 
that  for  a  lead  button  weighing  20  grams  and  containing  20  mgs. 
of  silver  (o.i  per  cent.),  the  temperature  required  to  keep  the 
button  molten  ranges  from  327°  C.  to  303°  C.,  until  the  button 


74 


A  MANUAL  OF   FIRE  ASSAYING 


Fig.  39  -  Freezing  point  Curve,  Lead  -  S.ilver 


1062' 


CuO 
PblOO 

Fig.  40  -  Freezing  point  Curve,  Lead  -  Copper 

has  decreased  ff  in  weight  by  the  loss  of  Pb,  practically  the 
entire  time  of  cupellation. 

When  the  button  has  reached  ^  of  its  original  weight,  the 
temperature  required  to  keep  it  molten  will  rapidly  increase, 
according  to  the  curve,  as  more  lead  is  oxidized,  until,  in  order 
to  prevent  freezing  and  get  pure  silver,  a  temperature  of  926°  C. 
and  slightly  above  must  finally  be  reached.  In  order,  however, 


CUPELLATION  75 

to  cause  a  rapid  formation  of  PbO  and  its  ready  absorption  by 
the  cupel,  and  not  have  heavy  losses  of  Au  and  Ag,  it  is  found 
that  a  temperature  of  about  700°  C.  is  best  for  the  main  part  of 
the  cupellation.  It  is  evident,  however,  that,  in  order  to  finish 
the  cupellation,  the  heat  must  be  raised  toward  the  end,  other- 
wise the  alloy  of  lead  and  silver,  as  it  increases  in  silver  percen- 
tage, will  tend  to  freeze,  i.e.,  to  solidify.  It  is  also  to  be  noted, 
however,  that  this  tendency,  with  most  lead  buttons  of  ordinary 
silver  contents,  is  not  reached  until  very  near  the  end  of  the 
cupellation.  It  is  an  old  saying  amongst  assayers  that  "a  cool 
drive  and  a  hot  blick"  are  essential  to  a  good  cupellation.  In 
the  cupellation  for  silver  it  would  seem  at  first  sight  that  a  final 
temperature  of  962°  C.  is  necessary  in  order  to  prevent  freezing 
and  to  obtain  a  silver  bead  free  from  lead.  However,  the  phe- 
nomenon of  the  "surfusion"  of  the  silver,  i.e.,  silver  in  a  molten 
state  below  its  true  melting-point,  due  probably  to  its  formation 
from  its  lead  alloy  by  the  oxidation  of  the  lead,  appears  to  indicate 
that  this  temperature  is  not  necessary.  It  is  true,  nevertheless, 
that  the  finishing  temperature,  depending  somewhat  upon  the 
amount  of  silver  present,  may  not  fall  much  below  750°  C.1 

It  is  plain  that  buttons  may  be  cupeled  at  temperatures  much 
above  those  stated,  but  the  loss  of  silver  and  gold,  both  by  ab- 
sorption and  volatilization,  is  very  much  increased  with  the 
higher  temperatures. 

The  reasoning  outlined  for  silver  applies  also  to  gold,  except 
that,  owing  to  the  somewhat  higher  melting-point  of  gold 
(1063°  C.),  the  finishing  temperature  should  be  a  little  higher. 

When  the  lead  buttons  are  contaminated  with  base  metals, 
such  as  copper,  the  temperature  of  cupellation  must  be  higher 
in  order  to  prevent  freezing.  The  reason  for  this  is  readily 
apparent  when  the  freezing-point  curve  (see  Fig.  40)  of  the  lead- 
copper  series  of  alloys  is  inspected.  The  freezing-point  of  an 
alloy  containing  10  per  cent.  Cu  and  90  per  cent.  Pb  is  900°  C. 

While  the  original  copper  percentage  in  the  lead  button  may 

1  The  subject  of  cupellation  offers  a  field  for  investigation.  The  actual  tem- 
perature of  cupellation  has  never  been  determined.  Due  to  the  active  oxidation 
of  the  lead  in  the  cupel,  it  is  higher  than  that  of  the  muffle,  where  the  cupel  stands. 
A  determination  of  the  temperature  of  the  cupeling  lead  will,  in  the  author's 
opinion,  very  much  modify  the  present  theory. 


76  A  MANUAL  OF   FIRE  ASSAYING 

be  quite  small,  the  copper  does  not  oxidize  as  readily  as  the  lead, 
and  tends  to  concentrate  in  the  button,  rapidly  raising  the 
melting-point  of  the  alloy. 

For  the  removal  of  copper  in  cupellation  the  ratio  of  Pb  to 
Cu  should  be  at  least  200  to  i  or  more.  Even  then  Cu  will  be 
retained  by  the  silver  and  gold  in  small  amounts.  If  it  is  less 
than  this,  considerable  copper  is  very  apt  to  be  retained  with  the 
silver  and  gold.  In  order  to  cupel  at  all,  the  ratio  of  Pb  to  Cu 
must  be  at  least  20  to  i.  In  general,  buttons  to  be  cupeled 
should  be  free  from  base  metal  impurities.  If  they  are  unavoid- 
ably present  in  the  button  from  the  crucible  assay,  the  base 
metals  should  be  removed  by  scorification  before  cupellation. 

Impurities  in  lead  buttons  are  detected  by  the  behavior  of 
the  button.  Zn,  As,  Sb,  and  S  tend  to  make  the  button  brittle 
when  hammered;  iron  and  copper,  etc.,  tend  to  make  it  hard. 
PbO  in  the  lead  button  makes  it  brittle.  PbO  is  often  found  in 
lead  buttons  that  have  been  produced  at  too  low  a  temperature. 
Where  the  gold  and  silver  contents  of  the  lead  button  approach 
30  per  cent,  of  its  weight,  it  is  brittle. 

However,  impurities  in  the  lead  button  will  not  always  be 
indicated  by  brittleness  or  hardness;  without  these  characteristics, 
impurities  may  still  be  present  in  sufficient  amount  to  cause  loss. 
All  impurities  do  not  cause  like  amounts  of  loss  in  cupellation. 
The  loss  due  to  the  presence  of  impurities  is  chiefly  in  absorption 
by  the  cupel,  and  comparatively  small  by  volatilization. 

The  accompanying  table  1  shows  the  influence  of  impurities. 
Twenty-five-gram  lead  buttons  were  cupeled,  containing  i  gram 
of  the  impurity  specified,  4  mgs.  of  Ag,  and  i  mg.  of  Au.  The 
temperature  of  cupellation  was  1000°  C,  in  order  to  prevent  freez- 
ing as  a  result  of  impurity. 

The  high  losses  are  due  in  part  to  the  high  temperature  em- 
ployed. The  table  really  gives  the  relative  influence  of  the 
impurities.  Bismuth  has  been  used  in  place  of  lead  for  cupellation. 

While  in  the  table  bismuth  is  stated  to  be  the  cause  of  a  very 
heavy  absorption,  this  is  not  substantiated  by  other  researches.2 
When  it  is  present  in  the  lead  button  it  tends  to  concentrate  dur- 

»  T.  K.  Rose,  in  "  Journ.  Chem.  Met.  and  Min.  Soc.  of  S.  A.,"  Jan.,  1905. 
2  K.  Sander,  "Berg  und  Huttenmnnisiiche  Zeitung,"   1903,  p.  81.     See  also 
"Min.  Ind.,"  XII,  p.  244 


CUPELLATION 


77 


ing  the  cupellation,  and  is  removed  by  oxidation  toward  the  last 
of  the  operation.  Some  of  it  is  very  apt  to  be  retained  by  the  pre- 
cious metal  bead.  Cupellation  may  be  carried  on  with  bismuth, 
but  the  absorption  is  much  higher.1  The  presence  of  Bi  in  the  cold 
cupel  may  be  recognized  by  the  fact  that  the  place  which  the  silver 
button  occupies  is  brown  and  surrounded  by  concentric  rings  of  a 
yellow  and  blackish-green  color.  Copper  colors  the  cupel  from  a 
dirty  green  to  a  black  color,  dependent  on  the  amount  of  copper. 

TABLE   XIII.  —  INFLUENCE   OF   IMPURITIES 


IMPURITY 

Loss  or  GOLD 

Loss  OF  SILVER 

REMARKS 

None  

1.2  per  cent. 

1  1.  8  per  cent. 

Tin  

2.0  per  cent. 

13.9  per  cent. 

\rsenic 

3.9  per  cent. 

16.3  per  cent. 

Antimony  
Zinc  
Cadmium  
Iron  
Manganese  
Molybdenum 

5.3  per  cent. 
9.3  per  cent. 
3.5  per  cent. 
4.0  per  cent. 
13.6  per  cent, 
n.o  per  cent. 

13.3  per  cent. 
17.6  per  cent. 
13.1  per  cent. 
1  6.6  per  cent. 
24.3  per  cent. 
26.2  per  cent. 

Most  of 
this  loss, 
even  with 
Te  and  Se, 
is  cupel 
absorption. 

Vanadium  

7.7  per  cent. 

21.7  per  cent. 

Copper  
Bismuth  2 

10.0  per  cent. 
21.8  per  cent. 

32.6  per  cent. 
27.9  per  cent. 

Thallium  
Tellurium  
Selenium 

23.1  per  cent. 
55.8  per  cent. 
154.1  per  cent. 

34.4  per  cent. 
67.9  per  cent. 
64.5  per  cent. 

Tin,  arsenic,  zinc,  cadmium,  iron,  and  manganese  cause  scoria 
to  form  on  the  cupel,  due  to  the  formation  of  oxides  which  are 
not  readily  absorbed.  Iron  causes  a  dark  coloration  of  the  cupel. 
Antimony  in  considerable  quantity  causes  the  cupel  to  check 
and  crack.  The  same  may  be  said  of  copper. 

Copper. — This  metal  is  oxidized  with  more  difficulty  than  lead, 
the  Cu2O  forming  by  aid  of  the  action  of  PbO;  however,  Cu2O, 
again  coming  into  contact  with  metallic  lead,  is  reduced  to  Cu, 
and  in  this  way  is  persistent  toward  the  end  of  the  cupellation, 
although  a  large  excess  of  Pb  over  Cu  is  present,  and  finally  some 
remains  with  the  Au  and  Ag.  The  loss  of  silver  during  the  cupel- 
lation is  due  mainly  to  absorption,  in  large  part  as  oxide.  This 

1  Smith,  in  "  Journ.  Chem.  Soc.,"  1894,  p.  863.  2  Doubtful. 


A   MANUAL  OF   FIRE  ASSAYING 


oxidation  of  the  silver  in  the  presence  of  much  lead  is  not  to  be 
ascribed  to  the  action  of  atmospheric  oxygen,  but  rather  to 
"oxygen  carriers,"  such  as  PbO,  Cu2O,  etc.  It  is  very  probable 
that  Cu2O  acts  peculiarly  in  this  manner,  and  the  high  absorption 
noticed  when  Cu  is  present  is  due  to  this  fact.  It  is  to  be  noted 
that  losses  in  silver  occur  toward  the  end  of  the  cupellation,  and 
occur  in  great  part  just  before  finishing;  the  small  dark  black-green 
rings,  surrounding  the  place  where  the  silver  bead  rests,  locates 
most  of  the  silver.  It  is  the  concentration  of  the  copper,  silver,  and 
gold  that  causes  the  high  absorption.  Lodge  1  shows  the  influence 
of  small  amounts  of  copper  on  the  cupellation  of  silver  and  gold. 

TABLE  XIV.  — COPPER  IN  CUPELLATION  OF  SILVER  AND    GOLD 


SILVER 

LEAD 

COPPER 

PERCENT- 
AGE OF 

TEMPER- 
ATURE 

PERCENT- 

RATIO PB  TO 

MILLI- 
GRAMS ' 

GRAMS 

GRAMS 

COPPER  IN 
LEAD 

DEGREES 

CENTI- 

AGE   OF 
Loss 

Cu 

GRADE 

202 

IO 

O.OIOI 

0.  I 

775 

1.05 

1000  to  i 

203 

10 

0.0202 

0.2 

775 

i.oS 

500  to  i 

202 

10 

o.  0303 

o-3 

775 

1.29 

333  to  i 

2O2 

10 

o  .  0404 

0.4 

775 

i-45 

250  to  i 

204 

10 

0.0500 

o-5 

775 

Cu  re- 

200 tO   I 

tained 

GOLD 

MILLI- 
GRAMS 

LEAD 

GRAMS 

PERCENT- 
AGE OF 
COPPER 

TEMPER- 
ATURE 

DEGREES 

CENTI- 

PERCENTAGE OF  Loss 

RATIO  PB  TO 
Cu 

IN  PB 

GRADE 

2O2 

IO 

no. 

775 

°-iSS 

202 

10 

0.  I  • 

775 

o.  19  '2  All  contained 

1000  to  i 

201 

TO 

0.2 

775 

o  .  20  copper  on 

500  to  i 

2OO 

10 

0-3 

775 

o.  13  finishing. 

250  to  i 

201 

10 

0.4 

775 

o.  165 

250  to  i 

202 

10 

°-5 

775 

o.  250 

200  tO   I 

1  "Notes  on  Assaying,"  p.  143  el  al. 

'2  Actual  losses;  copper  retained,  0.16  per  cent.     Gold  about  the  same  weight 
as  before  cupellation. 


CUPELLATION 


79 


Gold  is  more  retentive  of  copper  than  silver.  It  is  to  be  noted 
that  even  with  a  ratio  of  200  Pb  to  i  Cu,  it  is  not  possible  to 
remove  all  copper,  and  beads  obtained  from  mattes  and  heavy 
copper  ores  should  be  examined  for  copper;  otherwise  silver 
results  may  be  high.  Retained  copper  in  these  silver  beads  will 
compensate  for  loss  of  silver,  but  the  amount  retained  is  so  variable 
that  this  error  cannot  be  considered  to  compensate  the  loss. 

Tellurium.  —  Tellurium  has  a  great  affinity  for  gold  and  silver, 
and  if  present  in  an  ore  in  any  appreciable  amount,  some  of  it 
will  go  into  the  lead  button  with  the  gold  and  silver,  and  thus 
have  its  influence  on  the  cupellation.  It  tends  to  concentrate 
during  the  cupellation  and  is  with  difficulty  removed  by  oxidation. 
When  there  is  present  in  the  lead  button  more  than  15  per  cent, 
of  the  gold  and  silver  weight  in  tellurium,  the  beads  resulting 
from  cupellation  have  a  dull  and  frosted  appearance.  Larger 
amounts  than  this  cause  the  beads  to  divide  and  split  up  in  the 
cupel.  F.  C.  Smith1  shows  the  influence  of  tellurium  'on  the 
cupellation  as  follows,  these  results  being  confirmed  by  J.  C. 
Bailar  2  and  others. 

TABLE  XV.  —  TELLURIUM  IN  CUPELLATION  OF  GOLD  AND  SILVER 


CONTAINING 

Loss  BY  ABSORPTION 

Loss  BY  VOLATILI- 

MGS. OF 

MGS.  TE 

ZATION 

Au 

Ag 

Au 

Ag 

Au 

Ag 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

29.8 

24.76 

5-°4 

5-° 

13-44 

27.08 

5-65 

0.69 

28.45 

23.64 

4.81 

i5-° 

34-22 

35-78 

5.28 

i-75 

22.17 

18.42 

3-75 

15.0 

29.85 

32.01 

Tl  .92 

17-95 

CUPELED  WITH  12  GRAMS  or  LEAD 

Note  the  similar  effect  of  selenium. 

Antimony.  —  The  presence  of  antimony  causes  increased  losses 
by  absorption,  although  its  effect  is  not  as  pronounced  as  that 

luThe  Occurrence  and  Behavior  of  Tellurium  in  Gold  Ores,"  etc.,  in  Trans. 
A.  I.  M.  E.,XXVI,  p.  495. 

'2  "West.  Chem.  and  Met.,"  I,  p.  119. 
3  Selenium  instead  of  tellurium. 


8o  A  MANUAL  OF   FIRE  ASSAYING 

of  copper  or  tellurium.  During  the  cupellation  litharge  and 
antimony  combine  to  form  antimoniate  of  lead,  which,  if  present 
in  considerable  amount,  may  cause  the  formation  of  scoria  on 
the  cupel.  Small  amounts  of  antimony  tend  to  remain  with  the 
gold  and  silver,  as  with  copper  and  tellurium. 

As  a  guide  in  cupellation,  the  following  scale  of  color  temper- 
atures is  given.1 

DEGREES  CENTIGRADE 

Lowest  red  visible  in  the  dark    470 

Dark  blood-red  or  black-red 532 

Dark  red,  blood-red,  low  red 566 

Dark  cherry-red 635 

Cherry-red,  full  red 746 

Light  cherry,  light  red    843 

Orange 900 

Light  orange 941 

Yellow 1000 

Light  yellow 1080 

White 1205 

i  White  and  Taylor,  in  Trans.  Am.  Soc.  Mch.  Eng.,  XXI,  p.  628.     H.  M. 
Howe,  in  "Eng.  and  Min.  Journ.,"  LXIX,  p.  75. 


VIII 


PARTING 

PARTING  is  the  separation  of  gold  from  silver  by  means  of 
acid.  In  assaying,  nitric  acid  is  almost  exclusively  used,  although 
sulphuric  acid  may  be  employed.  In  order  to  separate  silver 
from  gold  by  means  of  acid,  it  is  essential  that  there  be  present 
at  least  twice  as  much  silver  as  gold.  When  less  silver'  is  present, 
it  is  impossible  to  separate  all  of  the  silver  from  gold  by  means 
of  acid  (see  assay  of  gold  bullion,  in  Chapter  XII).  When  the 
above-stated  amount  is  present,  it  requires  acid  of  not  less  than 
1.26  specific  gravity,  boiling  for  at  least  20  or  30  minutes,  to 
separate  the  silver  from  gold.  The  ratio  of  2  and  2.5  to  i  is 
used  practically  only  in  bullion  assay.  ' 

In  parting  beads  from  ore  assays,  it  is  considered  necessary 
to  have  at  least  five  times  as  much  silver  as  gold  present.  The 
addition  of  silver  to  gold  or  to  the  gold-silver  alloy  in  order  to 
prepare  for  parting  is  termed  "inquartation,"  from  the  fact  that 
at  least  3  parts  of  silver  to  i  part  of  gold  were  formerly  con- 
sidered necessary.  The  nitric  acid  used  for  parting  must  be  free 
from  hydrochloric  acid  and  chlorine  in  order  not  to  have  a  solvent 
action  on  the  gold.  Nitric  acid  should  be  examined  for  chlorides 
before  being  used  for  parting.  In  order  to  part  silver  from  gold 
successfully,  the  following  points  must  receive  careful  considera- 
tion: (i)  The  strength  of  the  acid  used;  (2)  The  temperature  of 
the  acid;  (3)  The  ratio  of  gold  to  silver  in  the  bead  to  be  parted. 

i.  The  proper  strength  of  acid  is  of  great  importance.  For- 
merly, most  authorities  recommended  that  acids  of  1.16  and  1.26 
sp.  gr.  respectively  —  2  parts  water  to  i  of  acid  (1.42  sp.  gr.) 
and  i  of  water  to  i  of  acid  —  be  used,  first  the  weak  acki  and  then 
the  stronger  acid.  T.  K.  Rose  recommends  4  parts  acid  to  3 
parts  water,  which  strength,  if  the  acid  be  heated,  will  not  break 
up  the  gold  in  the  bead  into  fine  particles,  even  if  50  parts  of 

81 


82  A   MANUAL  OF   FIRE  ASSAYING 

silver  are  present  to  i  part  of  gold.  Gold  is  less  apt  to  break  up 
when  it  is  less  than  o.io  mg.  in  weight.  Keller1  recommends 
acid  of  the  following  strength :  i  part  acid  (sp.  gr.  i  .42)  to  9  parts 
distilled  water.  In  this  strength  of  acid  the  gold  almost  invariably 
remains  in  a  coherent  mass,  even  when  the  silver  is  500  times  as 
much  as  the  gold.  This  is  the  strength  of  acid  recommended  for 
ordinary  assay  purposes.  The  beads  should  be  boiled  in  the  acid 
for  at  least  10  to  15  minutes  in  order  to  insure  parting. 

2.  It  is  essential  to  have  the  acid  at  the  boiling-point  before 
dropping  in  it  the  bead  to  be  parted.     Putting  the  bead  into  cold 
acid  and  heating  up  gradually  is  almost  certain  to  leave  the  gold, 
especially  where  the  ratio  of  silver  to  gold  is  high,  in  a  powdered, 
fine  condition,  very  apt  to  cause  losses  in  washing  and  subsequent 
handling  of  the  gold.     Gold  acid  should  not  be  used. 

3.  While  the  best  ratio  of  silver  to  gold,  for  parting  ordinary 
beads,  is  5  to  i,  this  ratio  is  not  always  under  control,  since  the 
assayer  must  be  content  in  many  cases  with  the  ratio  that  the 
ore  furnishes  him,  when  this  is  more  than  5  to  i.     If  less  than 
5  to  i,  silver  should  be  added  in  order  to  bring  it  up  to  this  ratio. 
The  silver  may  be  added  directly  to  the  crucible  or  scorification 
fusion,  or  to  the  lead  button  during  cupellation  when  it  is  not 
essential  to  determine  the  silver  in  the  ore. 

Where  it  is  essential  to  determine  the  silver,  and  inquartation 
is  necessary,  the  bead  from  the  cupellation  is  first  weighed,  the 
requisite  amount  of  silver  is  added  to  the  bead,  both  wrapped  up 
in  about  2  grams  of  sheet  lead,  and  then  it  is  recupeled  and  parted. 

Beads  which  need  inquartation  may  also  be  fused  with  silver, 
on  a  piece  of  charcoal,  by  means  of  the  blowpipe;  but  this  method 
is  not  to  be  recommended,  as  it  frequently  occasions  loss. 

Many  assayers,  where  they  suspect  an  ore  to  be  deficient  in 
silver  for  parting,  add  silver  to  the  crucible,  not  determining  the 
silver  in  this  assay,  but  running  a  separate  scorification  assay  for 
this  purpose. 

After  parting,  the  acid  is  poured  from  the  parting  cup  or 
flask  in  which  the  operation  has  been  conducted,  and  the  gold 
residue  is  washed,  at  least  three  times,  with  warm  distilled  water 
in  order  to  remove  all  trace  of  silver  nitrate.  The  black  stain 
occurring  in  parting  cups  after  heating  for  the  annealing  of  the 

1  Keller,  Trans.  A.  I.  M.  E.,  XXXVI,  p.  3. 


PARTING 


FIG.  41.  —  PARTING  BATH 

gold  is  due  to  metallic  silver  reduced  from  silver  nitrate  by  the 
heat,  showing  insufficient  washing.  Parting  may  be  carried  on 
in  small  porcelain  crucibles  called 
"parting  cups,"  or  in  test  tubes,  or 
in  flasks  similar  to  copper-assay 
flasks.  In  order  to  part  in  flasks 
or  test  tubes,  it  is  essential  to  have 
the  gold  stay  as  a  coherent  mass, 
so  as  to  prevent  loss  in  transference. 
When  parting  cups  are  used,  after 
washing,  the  gold  is  carefully  dried 
and  the  gold  annealed  at  a*dull-red 
heat,  either  in  the  muffle  or  by 
means  of  the  blowpipe.  After  acid 
treatment,  the  gold  is  left  as  a  soft 
black  mass,  probably  an  allotropic 
condition  of  the  gold;  but  upon 
heating  this  is  changed  to  the  nor- 
mal yellow  metallic  state  in  which 
it  is  weighed.  Fig.  41  shows  a  con- 
venient parting  bath  with  test  tubes;  Fig.  42  shows  parting  flasks 
commonly  in  use. 


FIG.  42.  —  PARTING  FLASKS 


IX 


THE  ASSAY  OF  ORES  CONTAINING   IMPURITIES 

IMPURITIES,  from  the  assayer's  point  of  view,  are  such  sub- 
stances, contained  in  ores,  furnace  products,  or  other  material, 
as  necessitate  some  particular  method  of  assay  or  treatment,  or 
the  observing  of  special  precautions  not  included  in  the  ordinary 
crucible  assay  as  already  outlined. 

Common  impurities  are  sulphur,  arsenic,  tellurium,  antimony, 
zinc,  copper,  etc.  Of  these  sulphur  is  by  far  the  most  common. 

In  performing  an  assay  it  is  usually  the  aim  of  the  assayer, 
whenever  this  is  possible,  to  produce  by  direct  fusion,  either  by 
the  crucible  or  scorification  method,  a  pure  lead  button  weighing 
approximately  20  grams.  If  the  button  is  smaller  than  this, 
there  is  danger  of  not  collecting  the  values;  if  larger,  cupellation 
is  too  prolonged  and  losses  are  increased.  The  impurities  men- 
tioned affect  either  the  size  of  the  button,  or  the  purity  of  the 
button,  or  both.  To  show  the  effect  of  sulphur  the  following 
definite  example  is  taken. 

Given  an  ore  containing  pyrite,  which,  in  a  charge  yielding 
the  ordinary  type  of  monosilicate  slag,  gives  a  reducing  power 
of  5  grams  of  lead  per  gram  of  ore.  If  the  following  charge, 

15  grams  of  ore  70  grams  of  PbO 

30  grams  of  Na2CO3  8  grams  of  SiO2 

Borax  glass  cover 

be  made  up  and  fused,  a  6o-gram  button  (approximately)  will  be 
produced,  on  top  of  which  will  be  a  small  quantity  of  "matte," 
i.e.,  an  artificial  sulphide  of  the  metals,  in  this  case  iron  and 
lead.  This  matte  is  brittle  and  may  contain  some  values.  On 
hammering  the  button,  it  is  lost.  In  general,  it  is  an  undesirable 
product  to  make.  A  small  amount  of  matte  is  produced  in  this 
case,  since  the  ore  has  the  power  to  reduce  75  grams  of  lead  from 
PbO,  while  only  70  grams  of  PbO  are  present,  so  that  the  excess 

84 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES          85 

sulphide  of  the  ore  not,  acted  upon  by  the  PbO  remains  in  the 
charge,  uniting  with  some  of  the  lead  to  form  a  sulphide  of  iron 
and  lead.  The  button  is  also  much  too  large  to  cupel.  If  in 
the  charge  the  PbO  is  materially  increased,  the  ore  will  react  to 
the  extent  of  its  full  reducing  power,  a  lead  button  of  75  grams 
will  be  produced,  no  matte  will  be  found,  and  the  slag  will  be 
improved,  owing  to  the  addition  to  it  of  the  fusible  base  PbO. 
If  the  PbO  in  the  charge  be  materially  reduced,  the  lead  button 
will  be  much  smaller  (owing  to  the  dearth  of  PbO  available  for 
reduction),  considerable  matte  will  be  formed,  and  the  slag  will 
be  poor. 

If  the  silica  be  increased,  so  that  sufficient  be  present  to  form 
the  higher  silicates  with  all  the  bases  present,  practically  no 
lead  will  be  reduced,  for  the  sulphide  has  not  the  power  to  reduce 
Pb  from  lead  soda  silicates  unless  a  free  base  be  present,  i.e., 

(PbO.Na2O)2SiO2  +  FeS2  (no  action), 
or  possibly 

(PbO.Na2O)2SiO2  +  FeS2  =(FeO.Na2O)2SiO2  +  PbS  +  S. 

In  this  way  the  sulphur  remains  in  the  charge  in  the  form  of 
sulphide  sulphur. 

Soda  will  cause  the  formation  of  SO3,  if  PbO  is  present  to 
furnish  the  oxygen,  and  if  it  can  act  as  a  free  base,  i.e.,  if  it  is 
not  combined  with  silica  (see  Chapter  V,  on  Reduction  and 
Oxidation  Reactions).  An  increase  of  soda  without  an  increase 
of  PbO  or  SiO2  will  lessen  the  amount  of  matte,  as  sulphur  will 
tend  to  combine  to  some  extent  with  the  Na2O  to  form,  with  the 
FeS,  a  double  sulphide  of  iron  and  soda,  etc.,  which  will  be  dis- 
solved in  the  slag.  The  above  outlines  the  effect  of  such  impu- 
rities as  sulphur  and  arsenic,  and  shows  the  necessity  of  special 
methods  of  assay  directed  toward  the  getting  rid  of  impurities. 

The  impurities  mentioned  may  be  divided  into  two  classes: 

(a)  Those  which  can  be  volatilized  by  oxidation  or  otherwise, 
e.g.,  sulphur,  arsenic,  and  antimony. 

(b)  Those  which  cannot  be  volatilized,  e.g.,  copper,  zinc,  etc. 
Some  of  these  may  be  partly  volatilized,  as  antimony  and 

zinc.     For  the   removal  of  all   of  them,   however,   whether  by 
volatilization  or  by  slagging,  oxidation  is  essential. 

In  one  method  employed  on  light  sulphide  or  arsenic  ores, 


Crucible  Fusions. 


86  A  MANUAL  OF   FIRE  ASSAYING 

the  iron-nail  method,  sulphur  and  arsenic  are  carried  into  the 
slag  as  a  double  sulphide  or  arsenide  of  soda  and  iron,  etc. 

The  following  methods  are  standard  methods  for  the  assay 
of  impure  ores,  and  are  discussed  in  detail: 

1.  The  roasting  method. 

2.  The  niter  method. 

(a)  The  common  niter  method. 

(b)  Miller's  oxide  slag  method. 

(c)  Perkins'  excess-litharge  method. 

3.  The  iron-nail  method. 

(a)  The  niter-iron  method. 

4.  The  cyanide  method  (rarely  used). 

5.  The  scorification  method. 

6.  The  combination  wet-and-dry  method 

(removal  of  impurities  by  solution). 

The  Roasting  Method.  —  It  is  usual  to  carefully  weigh  out 
0.5  or  i  assay  ton  of  the  ore  to  be  assayed,  and  place  it  in  a  roasting 
dish  of  sufficient  size  to  permit  of  stirring  without  loss  by  spilling. 
The  dish  is  placed  in  the  muffle,  the  temperature  of  which  is  not 
above  a  "black  red"  and  the  firing  of  which  is  under  good  control, 
so  that  the  temperature  will  not  rise  too  rapidly.  In  the  case  of 
an  ordinary  sulphide  ore,  such  as  a  pyrite,  or,  for  example,  a 
chalcopyrite  and  quartz,  the  following  reactions  take  place,  if 
the  roasting  is  carried  on  slowly  at  a  low  heat: 

3CuFeS2  +  i8O  +  heat  =  Cu2S  +  3FeSO4  +  CuSO4  +  SQ2 

At  590°  C.  the  ferrous  sulphate  decomposes  spontaneously, 
sulphatizing  the  balance  of  the  copper. 

Cu2S  +  2FeSO4  +  6O  =  2CuSO4  +  Fe2O3  +  SO3 

At  655°  C.  the  copper  sulphate  decomposes  into  basic  sulphate 
and  SO3,  and  at  700°  C.  into  CuO  and  SO3,  as  follows: 

2CuS04  =  CuO.CuS04  +  S03, 
CuO.CuS04  =  2CuO  +  S03; 

so  that  the  final  products  of  the  roast,  when  carried  to  above 
700°  C.,  are  ferric  and  cupric  oxide,  with  a  complete  removal  of 
the  sulphur.  If  the  temperature  is  not  carried  above  700°  C., 
sulphur  remains  in.  the  charge  as  sulphate,  which  may  again  be 
reduced  in  the  crucible  to  sulphides. 


THE    ASSAY   OF  ORES  CONTAINING    IMPURITIES          87 

2CuSO4  +  3C  =  Cu2S  +  SO2  +  3CO2 

If,  for  any  reason,  it  is  not  desirable  to  carry  the  temperature 
as  high  as  700°  C,  the  ore,  after  roasting  until  no  further  smell 
of  SO2  is  discernible,  is  cooled  and  mixed  with  5  to  10  grams  of 
powdered  (NH4)2CO3,  and  reroasted  at  a  low  heat,  the  sulphuric 
anhydride  (SO3)  being  eliminated  as  volatile  ammonium  sulphate, 
(NH4)2S04. 

CuS04  +  (NH4)2C03  -  CuO  +  (NH4)2S04  +  CO2 

Any  silver  in  the  ore  that  has  been  roasted  will  be  in  the  form 
of  Ag2SO4,  or  if  arsenic  and  antimony  are  present,  partly  in  the 
form  of  arseniates  and  antimoniates.  If  the  roasting  temperature 
is  carried  to  870°  C.  and  above,  the  silver  sulphate  will  be  de- 
composed, leaving  the  silver  in  the  form  of  metallic  silver.  In 
order  to  avoid  loss  of  silver  it  is  best  not  to  carry  the  temperature 
above  700°  C. 

In  roasting  plain  pyrite  ores,  the  reactions  are  similar,  but 
simpler,  and  the  temperature  need  not  be  carried  above  600°  C. 
During  roasting,  the  ore  should  be  stirred  frequently  in  order  to 
expose  fresh  surfaces  to  oxidation. 

When  ores  contain  arsenic  and  antimony,  the  roasting  opera- 
tion is  more  difficult  and  complex,  and  considerable  care  and 
skill  are  required  to  eliminate  the  greater  part  of  these  two  volatile 
elements.  The  reason  for  this  is  that  the  arsenic  and  antimony 
pass  by  roasting,  first  to  the  state  of  the  lower  oxides  As2O3, 
Sb2O3,  which  are  volatile,  and  then  to  the  state  of  the  higher 
oxides  As2O5,  Sb2O5,  forming  arseniates  and  antimoniates  of  cer- 
tain metals  present  in  the  ore,  some  of  which  are  stable  even  at 
high  temperatures,  thus  fixing  the  arsenic  and  antimony  in  the 
roasted  ore,  and  not  eliminating  it.  The  arseniates  (or  anti- 
moniates) which  ordinarily  form  are  those  of  copper,  iron  and 
silver.  The  best  conditions  for  the  elimination  of  arsenic  and 
antimony  are  alternate  oxidation  and  reduction  at  a  low  heat. 
The  presence  of  sulphur  tends  to  aid  the  elimination  of  arsenic 
and  antimony  by  the  formation  of  the  volatile  sulphides  of  these 
metals.  The  reducing  action  necessary  for  the  elimination  of 
arsenic  and  antimony  is  best  obtained  by  mixing  with  the  ore 
equal  volumes  of  coal  dust  or  charcoal,  and  roasting  at  a  dark-red 
heat  until  the  coal  is  burnt  off,  then  cooling,  adding  more  coal 


88        .  A   MANUAL  OF   FIRE  ASSAYING 

dust,  and  reroasting.  In  this  way  the  greater  part  of  the  arsenic 
and  antimony  can  be  readily  volatilized,  except  in  very  rich 
silver  ores.  When  galena  ores  are  to  be  roasted,  the  ore  is  best 
mixed  with  an  equal  volume  of  silica  and  roasted  at  a  very  low 
heat.  In  this  roast  PbSO4  is  formed  to  a  considerable  extent, 
which  at  a  higher  heat  is  decomposed  by  the  SiO2  present,  as 
follows: 

PbSO4  +  SiO2  =  PbSiO3  +  SO3 

Care  must  be  taken  with  this  roast  as,  at  the  formation  point 
of  lead  silicate,  silver  losses  are  apt  to  occur.  A  successful  roast 
will  be  indicated  by  a  yellow  color  (lead  silicate),  and  an  unsuc- 
cessful one  by  a  black  or  gray  color  (fused,  undecomposed  sul- 
phides). In  general,  heavy  sulphide  ores  that  contain  their  chief 
value  in  gold  may  be  roasted,  when  this  is  carefully  done,  without 
loss  of  gold;  but  silver  ores,  especially  when  of  high  grade,  are 
apt  to  give  low  results. 

In  making  up  the  charge  for  the  roasted  ore,  it  is  to  be  noted 
that  from  a  sulphide  ore  (pyrite,  etc.)  the  product  is  frequently 
of  an  oxidizing  nature  and  basic,  which  must  be  taken  into  account 
in  adding  the  fluxes.  In  galena  ores,  when  silica  has  been  added, 
this  must  be  accounted  for. 

The  roasting  method  is  frequently  used  for  heavy  sulphide 
ores,  especially  when  they  have  a  low  value  in  gold  and  silver, 
as  it  permits  of  a  large  amount  of  ore  being  taken  (i  assay  ton 
and  more),  which  after  roasting  presents  no  difficulty  in  making 
the  proper  fusion. 

The  Niter  Method.  —  The  first  step  in  the  niter  method  is  the 
making  of  a  preliminary  assay  according  to  the  directions  already 
given.  The  precautions  concerning  the  reducing  power  of  the 
sulphides  in  different  types  of  charges  must  be  carefully  noted; 
it  is  best  to  have  the  preliminary  charge  of  the  same  composition 
as  the  final  assay  charge.  Or  else  the  reducing  power  may  be 
determined  by  the  soda-litharge  charge  and  this  cut  down  by 
25  per  cent.,  20  grams  deducted  for  the  lead  button,  and  the 
remainder  divided  by  4  to  get  the  amount  of  niter  to  add,  in 
grams,  if  the  monosilicate  slag  is  to  be  made  in  the  assay. 

The  amount  of  ore  taken  for  the  niter  assay  varies  according 
to  the  grade  of  the  ore  in  gold  and  silver  and  according  to  the 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES          89 

amount  of  impurity  present.  It  is  rarely  desirable  to  add  more 
than  20  grams  of  niter  to  the  charge,  as  larger  amounts  cause 
difficulty  through  the  evolution  of  too  much  gas.  One-half  assay 
ton  is  the  amount  of  ore  most  frequently  taken.  Sometimes, 
with  ores  containing  much  impurity,  o.  10  to  0.25  assay  ton  is 
used.  Twenty-gram  crucibles  (170  c.c.  capacity)  are  used  for 
amounts  of  0.5  assay  ton  of  ore  and  less,  and  3o-gram  crucibles 
(240  c.c.  capacity)  for  i  assay  ton  of  ore. 

Miller's  Oxide-Slag  Method.  —  This  method  is  a  modified  niter 
method  applicable  to  such  ores  as  contain  practically  no  silica; 
i.e.,  heavy  sulphide  ores,  such  as  pyrites,  arsenopyrite,  mattes, 
etc.  It  is  based  on  the  fact  that  PbO  has  the  power  to  hold  in 
solution  and  in  suspension  oxides  of  such  metals  as  copper,  iron, 
etc.  (see  p.  95,  where  "scorification"  is  discussed),  in  certain 
amounts.  Niter  is  added  to  oxidize  the  sulphides,  etc.,  and 
Na2CO3  to  aid  in  the  complete  oxidation  of  the  sulphur  by  the 
formation  of  sulphates,  in  the  manner  already  discussed.  The 
first  step,  as  in  the  ordinary  niter  method,  is  the  preliminary 
assay,  according  to  the  following  charge: 

Ore 3  grams 

PbO   50  grams 

Na2CO3 8  grams 

The  final  charge  is  as  follows:. 

Ore 0.5  assay  ton 

PbO   70 .  o  grams 

Na2COs    12.0  grams 

KNOs    (calculated    for   a    2o-gram 

button) 

Quick  fires,  1100°  C,  30  minutes,  are  found  to  be  best.  The 
slags  are  usually  dull  black  and  pour  readily,  and  the  button 
separates  easily  from  the  slag.  (In  slags  high  in  silica  or  con- 
taining much  borax,  the  lead  buttons  are  apt  to  adhere  closely 
to  the  slag.)  With  the  oxide-slag  method,  trouble  is  sometimes 
experienced  through  the  lead  refusing  to  collect  and  remaining 
shotted  through  the  slag.  The  difficulty  is  usually  due  to  too 
much  soda  (especially  if  considerable  niter  is  used),  although  too 
low  a  temperature  of  fusion  is  also  a  factor. 


90  A   MANUAL  OF   FIRE   ASSAYING 

The  method  gives  reliable  results  on  gold  and  silver,  comparing 
well  with  the  other  standard  methods.1 

Perkins  Excess-  Litharge  Method.2  -  -  This  method  is  based  on 
the  fact  that  PbO  will  dissolve  oxides  of  other  metals  and,  if 
present  in  very  great  excess,  will  prevent,  to  a  large  extent,  the 
reduction  of  other  metals,  such  as  Cu  and  Sb.  The  presence  of 
so  much  PbO  also  insures  a  strongly  oxidizing  tendency  in  the 
crucible,  preventing  impurities  entering  into  the  button. 

\  It  is  desirable  to  add  or  have  present  SiO2  in  such  an  amount 
asj  will  form  a  monosilicate  with  the  bases  present,  including 
scwfie  litharge,  but  leaving  much  litharge  uncombined  in  the 
charge. 

The  following  table  shows  the  proportion  of  PbO  required  to 
form  fusible  compounds  with  the  principal  metallic  oxides:3 

TABLE  XVI.—  PbO   REQUIRED   WITH  METALLIC    OXIDES 


One  part  of  .....     CuzO      CuO      ZnO      FesO4      Fe2O3      MnO      SnO2     Sb2Os 
Requires  parts  of  PbO    .1.5  1.8          8  4  10  10          13  5  i 

In  order  to  carry  out  the  excess-litharge  method  intelligently, 
it  is  necessary  to  know  the  approximate  composition  of  the  ore, 
so  as  to  provide  the  proper  amount  of  PbO  and  SiO2.  The  best 
fusion  exhibits,  in  a  section  of  the  cone  of  the  slag  after  breaking, 
silicates  of  lead,  iron,  etc.,  on  the  outer  surface,  gradually  passing 
to  crystalline  litharge  toward  the  center.  The  temperature  of 
fusion  should  not  exceed  1050°  to  1100°  C.  It  must  be  above 
906°  C.  (melting-point  of  PbO).  The  first  step  is  the  making  of 
a  preliminary  assay  in  order  to  determine  the  amount  of  niter 
to  be  added.4 

The  final  charge  most  frequently  used  is: 

Ore  ........   o.  25  to    0.5  assay  ton          Na2COs   ..................  12  grams 

PbO   .......   8        to  10      assay  tons         SiO2    ............  ."  ........  10  grams 

Niter  to  obtain  2o-gram  button 

The  button  is  generally  clean,  and  separates  easily  from  the 

1  Miller,  "The  Reduction  of  Lead  from  Litharge,"  etc.,  in  Trans.  A.  I.  M.  E., 
XXXIV,  pp.  398,  399. 

2W.  G.  Perkins,  "The  Litharge  Process,"  ibid.,  XXXI,  p.  913. 

3  Hofman,  "  Metallurgy  of  Lead,"  p.  7. 

4  In  place  of  niter,  it  may  be  necessary,  in  this  method  or  in  Miller's  method, 
to  add  argol,  if  ore  is  rot  reducing. 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES          91 

slag.     The  method  has  been  stated  to  give  low  results  in  silver, 
but  this  is  contrary  to  the  experience  of  the  author. 

The  Iron-Nail  Method.  —  This  method  does  not  attempt  to  ^ 
oxidize  impurities,  but  to  carry  sulphur,  etc.,  into  the  slag.  The 
ore  is  decomposed  by  the  iron  nails  added  to  the  charge  and  by 
the  PbO  present.  As  iron  reduces  PbO  to  Pb,  the  amount  of 
litharge  added  to  the  charge  is  limited  to  25  to  30  grams.  The 
amount  of  soda  needed  is  large,  as  this  flux  is  depended  upon  to 
carry  the  sulphur  into  the  slag.  The  slag  should  be  below  a 
monosilicate  in  degree,  and  high  in  soda,  as  basic  slags  have  a 
high  solvent  power  for  sulphides. 

A  typical  charge  on  an  ore  that  has  a  reducing  power  of  about 

4  grams  of  Pb  per  gram  of  ore  is.1 

Ore 0.5  assay  ton  SiC>2    2  grams 

NaHCOs 30  grams  borax    8  grams 

PbO   30  grams  nails 17  grams 

Salt  cover 

The  soda  should  usually  be  twice  the  amount  of  ore  in  the  charge. 
The  reactions  that  take  place  are  approximately  as  follows: 
;PbO  +  FeS2  +  4NaHCO3  =  ;Pb  +  2Na2SO4  +  FeO  +  4CO2  + 

2H20 

Part  of  the  ore  is  decomposed  by  the  PbO,  and  part  of  the  S 
may  go  off  as  SO2,  as  discussed  in  previous  pages.     The  iron 
nails  decompose  the  balance  of  the  sulphides. 
FeS2  +  Fe  =  2FeS 
PbS  +  Fe  =  Pb  +  FeS  (if  galena  is  present  or  lead  sulphide  forms). 

The  iron   sulphide   (FeS)   is  dissolved  by  the  alkaline  slag, 
forming  probably  double  sulphides  of  soda  and  iron. 

To  show  the  nature  of  the  iron-nail  fusion,   the  following 
results  of  two  fusions  on  a  pyrite  ore  containing  39.5  per  cent. 

5  —  a  reducing  power  equal  to  about  8  —  are  given : 2 

Charge  i  Charge  2 

i  assay  ton ore    0.5  assay  ton 

30  grams   NaHCOs 30  grams 

30  grams PbO 30  grams 

4  grams SiOo   4  grams 

4 .     nails 4 

10  grams borax  glass  cover  .  .  10  grams 

1  Lodge,  "Notes  on  Assaying,"  p.  99.  2  Lodge,  ibid.,  p.  101. 


A   MANUAL  OF   FIRE  ASSAYING 


The  following  results  were  obtained 


No. 


Slag    60  grams 

Matte 23.5  grams 

Lead 24 . 5  grams 

Crucible  and  charge  before  fusion    685  grams 

Crucible  and  charge  after  fusion 665  grams 

Loss  in  weight    20  grams 

Nails  before  fusion    64  grams 

Nails  after  fusion     43  grams 

Loss  of  iron    21  grams 

Per  cent,  of  S  in  slag 6 .  73 


No.  2 

65         grams 

none 

26.5     grams 


S  in  slag 

S  in  ore 

S  passed  off  as  SO2 

S  in  matte  . 


4.03  grams 

IT  .85  grams 

0.95  grams 

6.87  grams 


662 

642 

20 
63 

49 

14 
7-63 

4.96  grams 
5.92  grams 
0.96  grams 

none 


grams 
grams 
grams 
grams 
grams 
grams 


It  will  be  noted  that  the  charges  are  identical  as  far  as  the 
fluxes  are  concerned,  but  that  the  amount  of  ore  differs.  It  is 
desirable  in  heavy  sulphide  ores  to  keep  the  ore  down  to  0.5 
assay  ton  and  lower  if  necessary. 

Care  must  be  taken  not  to  have  the  slag  above  a  monosilicate 
in  degree,  for  if  higher  in  SiO2  there  will  be  particular  danger  in 
this  charge  of  not  having  the  sulphides  oxidized  by  the  PbO, 
more  sulphide  being  retained  in  the  charge  than  it  can  dissolve, 
and  forming  a  matte,  even  with  small  amounts  of  ore. 

The  Niter-Iron  Method.  —  This  method  is  in  principle  the 
same  as  the  iron-nail  method.  An  amount  of  niter  is  added  at 
random,  sufficient  to  oxidize  but  a  portion  of  the  sulphides,  the 
balance  being  decomposed  by  the  nails. 

The  Cyanide  Method.  —  Sometimes,  when  no  other  fluxes  are 
at  hand,  or  when  a  rapid  assay  is  to  be  made  in  which  accuracy 
is  not  essential,  a  fusion  of  ore  with  cyanide  may  be  made,  and 
the  resultant  button  cupeled  for  silver  and  gold.  The  method 
is  a  rapid  one  and  gives  good  malleable  buttons,  but  is  apt  to  be 
low  in  gold  and  silver,  especially  in  silver.  The  cyanide  used 
should  be  pure,  free  from  carbonates  or  other  impurities,  and 
the  fusion  should  be  made  at  a  low  temperature.  The  following 
charge  is  used: 

Ore o .  5  to  i  assay  ton 

PbO   25  grams 

KCN    .   3  assay  tons 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES        93 


When  the  ore  contains  copper  and  other  base-metal  impuri- 
ties, these  are  reduced  and  enter  the  lead  button.  Sulphur  is 
taken  up  by  the  slag  as  potassium-sulpho-cyanate  (KCNS).  In 
general,  it  is  a  method  not  to  be  recommended.  The  following 
results  show  the  loss  in  silver  which  takes  place  in  this  method.1 

TABLE   XVII.  — LOSS    OF   SILVER   IN    CYANIDE   METHOD 

NITER  METHOD       CYANIDE  METHOD 


Silver,  by  uncorrected  assay 

Silver  in  slag 

Silver  from  cupel   


4. 10  mgs. 
7.81  mgs. 


525.5    mgs. 

36.8    mgs. 

6.56  mgs. 


The  results  are  averages  of  duplicate  assays.  The  loss  of 
gold  in  the  slag  by  cyanide  fusion  is  not  nearly  so  marked  as  that 
of  silver. 

A  Comparison  of  ihe  Different  Crucible  Methods  of  Assay  for 
Impure  Ores.  —  In  very  impure  ores,  containing  large  amounts  of 
sulphur,  arsenic,  etc.,  the  roasting  method  is  applicable  when  gold 
only  is  to  be  determined,  or  when  silver  results  need  not  be  very 
accurate.  The  roasting  method  gives  uniformly  lower  silver  re- 
sults than  most  of  the  other  methods,  although  to  a  large  extent 
this  is  due  to  roasting  at  too  high  a  temperature. 

The  roasting  method  has  the  advantage  that  when  ores  are 
low  grade  large  quantities  of  ore  can  be  taken,  which  is  not  always 
possible  with  the  other  methods.  Roasting,  however,  must  be 
skilfully  conducted  in  order  to  be  successful. 

The  niter  method  is  a  desirable  and  clean  method  of  assay 
giving  accurate  results.  Where  large  quantities  of  niter  are  em- 
ployed, the  oxidizing  action  in  the  crucible  is  greatly  increased, 
and  it  is  probable  that  thereby  losses  in  silver  are  apt  to  occur 
by  the  slagging  of  the  silver. 

There  is  no  accumulated  evidence  on  this  subject,  but  many 
assayers  hold  this  opinion.  The  niter  method  is  desirable  for 
such  ores  as  do  not  contain  amounts  of  sulphur  requiring  ex- 
traordinary amounts  of  niter.  Usually,  the  limit  of  niter  in  a 
charge  is  placed  at  about  20  grams;  if  the  ore  should  require  more 

'E.  H.  Miller.  "Corrected  Assays,"  in  "  Sch.  Mines  Quart.,"  Vol.  XIX, 
November,  1897. 


94  A  MANUAL  OF   FIRE  ASSAYING 

than  this,  it  is  generally  considered  advisable  to  reduce  the  quan- 
tity of  ore  taken  for  the  assay.  This  has  the  disadvantage  of 
multiplying  the  error  of  the  assay  when  finding  the  value  per  ton. 

The  modified  niter  methods  discussed  offer  advantages  in  the 
slagging  of  base-metal  impurities.  This  is  particularly  true  of 
copper  and  zinc.  It  is  very  much  easier. to  cause  copper  to  enter 
the  slag  when  an  oxide  slag  is  made  than  when  a  silicate  is  made. 
This  is  partly  due  to  the  oxidizing  nature  of  the  high  litharge 
charges.  The  best  method  for  the  slagging  of  base-metal  impu- 
rities is  the  excess-litharge  method. 

The  iron-nail  method  is  a  standard  method,  which  can  be 
successfully  applied  to  most  sulphide  ores  and,  with  care,  to 
arsenical  ores.  It  is  not  applicable  to  ores  containing  base-metal 
impurities,  such  as  copper,  for,  being  essentially  reducing  in  its 
nature,  practically  all  of  the  base-metal  impurities  will  be  found 
in  the  lead  button.  When  used  with  arsenical  ores,  the  temper- 
ature employed  should  be  low,  not  above  1050°  C. ;  otherwise 
speiss  (an  artificial  arsenide  of  iron)  is  apt  to  form,  which  may 
carry  values.  It  also  has  the  objection,  in  the  case  of  very 
impure  ores,  that  small  quantities  must  be  taken  for  assay, 
involving  serious  risk  of  multiplying  an  error  of  assay. 

Scarification  is  the  oxidizing  fusion  of  ore  with  metallic  lead 
in  the  muffle-furnace,  producing,  in  the  main,  a  litharge  slag,  i.e., 
an  oxide  slag.  It  is  a  method  of  assay  which  requires  no  previous 
preparation  of  the  ore  or  preliminary  assay,  and  as  practically 
only  one  flux  is  employed,  it  is  both  a  cheap  and  a  rapid  method. 
It  is  also  a  thoroughly  reliable  method,  when  proper  precautions 
are  taken  and  when  it  is  employed  on  material  suitable  for  the 
purpose.  The  operation  is  performed  in  shallow  fire-clay  dishes, 
called  scorifiers. 

The  sizes  commonly  used  are: 

i  .  5-in.  scorifiers;  cubic  contents 15  c.c. 

2.o-in.  scorifiers;  cubic  contents 25  c.c. 

2 .5-in.  scorifiers;  cubic  contents 37  c.c. 

3. 5-in.  scorifiers;  cubic  contents 100  c.c. 

The  dimensions  referred  to  are  outside  dimensions.  The  size 
most  commonly  employed  is  the  2.^-m.  one.  Before  these  dishes 
are  used  it  is  usual  to  line  the  inside  with  ferric  oxide.  This  is 
done  by  preparing  crushed  iron  ore  or  ochre,  mixing  with  water, 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES        95 

and  painting  the  inside -of  the  dishes.  This  gives  them  a  basic 
lining,  and  to  some  extent  prevents  the  oxide  slag  from  attacking 
the  silica  in  the  clay. 

Some  scorifier  slags,  especially  if  they  contain  copper,  are 
very  corrosive.  The  amount  of  ore  taken  for  scorification  varies 
from  o.io  assay  ton  to  0.25  assay  ton;  but  o.io  assay  ton  is  the 
amount  most  frequently  taken.  The  larger  amounts  are  rarely 
used,  unless  the  ore  contains  practically  no  bases.  Sometimes, 
for  very  impure  material,  as  little  as  0.05  assay  ton  is  taken. 
The  amount  of  test  lead  varies  according  to  the  nature  of  the  ore. 
The  more  impure,  the  ore  the  larger  will  be  the  ratio  of  lead  to 
ore.  With  o.io  assay  ton  the  test  lead  will  vary  from  40  to  100 
grams.  A  common  charge  is  40  to  50  grams  of  test  lead  for 
ordinary  ores.  As  already  pointed  out,  certain  quantities  of 
litharge  are  required  in  order  to  make  fusible  compounds  with 
the  metallic  oxides.  If  the  ore  contains  small  amounts  of  the 
metallic  oxide,  the  test  lead  will  be  small  in  amount;  if  it  contains 
appreciable  quantities  of  ferric  oxide  (Fe2O3)  or  Cu,  etc.,  large 
amounts  of  test  lead  will  be  required.  It  is  best  to  add  a  small 
amount  of  borax  glass  to  the  charge,  from  i  to  1.5  grams,  scat- 
tering it  over  the  surface  of  the  lead.  This  aids  in  the  solution 
of  the  bases  present.  When  the  ore  contains  the  basic  oxides 
mentioned,  borax  glass  up  to  3  and  4  grams  will  materially  aid 
in  forming  good  slags,  without  infusible  scoria.  These  infusible 
scoria  often  appears  in  ores  containing  large  amounts  of  bases, 
and  is  very  apt  to  give  low  results  by  entangling  unfused  portions 
of  ore  within  itself.  It  is  best  to  mix  the  weighed-out  portion 
of  ore  with  one-half  of  the  test  lead  to  be  used,  and  then  cover 
over  with  the  balance. 

The  scorification  may  be  divided  into  the  following  distinct 
steps : 

1.  Melting.     In  this  stage  the  lead  melts,  and  the  ore,  being 
of  a  lesser  gravity,  rises  to  the  surface  of  the  molten  lead  and 
floats  there. 

2.  Roasting.     The  ore  on  the  surface  of  the  lead  is  attacked 
by  the  oxygen  of  the  air  and  roasts  in  the  same  way  as  described 
under  "Roasting  of  Ores." 

3.  Scorification    Proper.     The    lead    commences    to    oxidize, 
forming  litharge.     A  small   percentage   (5),   volatilizes  and   the 


96  A  MANUAL  OF   FIRE  ASSAYING 

balance  forms  a  fusible  slag.  This  now  absorbs  the  oxides  formed 
by  the  roasting,  dissolving  them  and  forming  an  igneous  solution. 
The  silver  and  gold,  liberated,  are  absorbed  by  the  remaining 
metallic  lead.  The  slag,  as  it  forms,  drops  to  the  side,  forming 
a  slag  ring,  with  the  center  of  the  lead  bath  open  to  the  atmos- 
phere. The  reason  for  this  is  that  the  meniscus  of  molten  lead 
is  convex,  thus  causing  the  collecting  of  the  slag  on  the  rim  of 
the  scorifier.  The  scorification  continues  until  the  whole  of  the 
lead  is  covered  over  with  slag.  It  is  then  considered  finished 
and  the  assay  is  poured.  Should  the  assay  be  left  in  the  muffle, 
the  lead  will  still  continue  to  oxidize,  although  none  is  exposed 
to  the  air,  the  interchange  of  oxygen  taking  place  by  means  of 
the  litharge  and  other  oxides  present.  The  size  of  the  lead 
button  desired  from  this  assay  ranges  from  15  to  20  grams.  If 
the  scorification  is  continued  to  produce  smaller  buttons,  losses 
are  apt  to  occur  by  oxidation  of  the  silver,  especially  if  this  is 
present  in  considerable  amounts,  thus  forming  rich  slags. 

The  temperature  of  scorification  ranges  from  1000°  C.  to  1 100° 
C,  although  with  pure  ores  higher  temperatures  may  be  employed. 

When  impure  ores  containing  much  base  metal  are  scorified, 
the  buttons  from  the  scorification  are  very  apt  to  be  contaminated 
with  base  metal,  especially  copper,  and  will  then  have  to  be 
rescorified,  with  more  test  lead,  in  order  to  get  a  pure  button  for 
cupellation. 

All  metals  are  to  some  extent  oxidized  simultaneously,  but  a 
mixture  of  metals  may  be  roughly  separated  by  successive  oxida- 
tion, each  metal  in  turn  partially  protecting  the  metal  next  in 
order,  while  the  latter  may  act  as  an  oxygen  carrier  to  the  former.1 
The  order  of  oxidation  is  as  follows: 

Fe  to  Fe2O3  Cu  to  Cu2O 

Zn  to  ZnO  Pt   to  — 

Ph  to  PbO  Ag  to  Ag2O 

Ni  to  Ni2O3  Au  to  AuO 

The  order  of  oxidation  of  the  following  elements  is  not  so  cer- 

Sb  to  Sb2O3  Bi  to  Bi2O3 

As  to  As2O3  Te  to  TeO2 

C    to  CO2  S     to  SO2 

1T.  K.  Rose,  "Refining  Gold  Bullion,  etc.,  with  Oxygen  Gas,"  in  Trans. 
I.  M.  M.,  April,  1905. 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES        97 

The  order  given  in  the  table  shows  the  difficulty  encountered 
in  the  removal  of  copper  by  scorification,  as  lead  stands  ahead 
of  it  in  the  order  of  removal,  and  it  is  very  difficult  and  requires 
a  number  of  re-scorifications,  if  the  amount  of  copper  is  large, 
to  reduce  it  to  such  an  amount  as  to  prevent  loss  in  cupellation. 
Iron  and  zinc  are  very  readily  removed  by  scorification  (oxida- 
tion). Certain  elements,  like  Te  and  Se,  are  difficult  to  remove 
from  the  lead  button,  and  may  tend  to  concentrate  with  the  Au 
and  Ag  in  the  final  cupellation. 

The  slag  from  the  scorification  assay  should  be  homogenous 
and  glassy.  If  it  has  an  earthy  appearance,  it  is  an  indication 
of  too  low  a  temperature  having  been  used,  and  the  button  is 
apt  to  be  brittle,  due  to  contained  PbO.  White  patches  of 
sulphate  of  lead  on  the  slag  after  pouring  also  indicate  rather 
too  low  a  temperature  of  scorification,  as  this  sulphate  forms  at 
a  low  temperature  under  slow  oxidation. 

The  scorification  method  is  a  reliable  one  on  most  materials, 
with  the  exceptions  enumerated  below.  As  the  usual  quantity 
taken  for  assay  is  o.  i  to  0.2  assay  ton,  it  is  evidently  not  a  suitable 
method  for  low-grade  ores,  especially  low-grade  gold  ores,  where 
at  least  0.5  to  i.o  assay  ton  must  be  taken  in  order  to  get  accurate 
results,  and  avoid  the  multiplication  of  the  error  of  weighing. 
It  is  practically  impossible  to  get  reliable  results  on  $5  to  $10 
gold  ores  by  ordinary  scorification.  If,  however,  10  assays  of 
o.i  assay  ton  are  made,  the  buttons  from  these  combined  and 
re-scorified  into  one  button,  which  is  then  cupeled,  the  results 
are  reliable,  but  not  so  good  as  from  the  crucible  assay  on  the 
same  total  amount,  on  account  of  the  multiplicity  of  weighing 
and  other  operations,  which  occasion  errors  and  losses.  The 
method  in  this  instance  would  also  be  more  costly  of  time  and 
materials. 

For  ordinary  and  rich  silver  ores,  and  very  rich  gold  ores  or 
furnace  products,  such  as  bullions,  mattes,  etc.,  the  method  is  a 
desirable  one.  It  requires  no  preliminary  operations  and  thus 
saves  valuable  time.  The  slag  loss  is  frequently  somewhat  higher 
than  in  the  crucible  assay.  It  is,  as  ordinarily  performed  (in 
duplicate),  a  cheap  method  as  regards  fluxes,  etc.  It  does  not 
give  good  results  on  very  basic  ores,  i.e.,  those  containing  hema- 
tite, manganese  oxides,  etc.,  as  in  this  case,  unless  a  great  deal 


98  A  MANUAL  OF   FIRE  ASSAYING 

of  lead  is  used,  scoria  are  apt  to  form  in  the  slag,  which  may 
entangle  lead  and  undecomposed  ore.  Neither  does  it  give  good 
results  on  telluride  ores,  zinc  precipitates,  or  ores  that  contain 
chloride  of  silver. 

When  basic  material  is  to  be  scorified,  small  additions  of  SiO2, 
up  to  i  gram,  may  prove  advantageous.  In  general,  however,  the 
addition  of  fluxes,  except  test  lead,  is  not  to  be  recommended. 
Scorification  may  be  modified  by  the  addition  of  considerable 
amounts  of  borax  glass,  litharge,  silica,  when  it  approaches  the 
crucible  assay  in  character  with  none  of  its  advantages. 

The  Combination  Method.  —  The  trouble  arising  from  the 
presence  of  considerable  amounts  of  base  metals,  such  as  copper 
and  zinc,  has  been  fully  discussed  in  previous  pages,  as  well  as 
the  difficulty  of  their  removal  by  fusion  methods.  For  this 
reason  the  combination  wet-and  dry-method  has  been  developecj, 
to  remove  the  objectionable  impurities  by  solution.  The  method 
is  used  chiefly  on  copper-bearing  material,  such  as  heavy  copper 
ores,  copper  mattes,  blister  copper,  and  to  a  lesser  extent  on 
zinc  ores,  and  on  cyanide  precipitates  produced  by  zinc,  and 
has  been  advocated  for  telluride  ores. 

Van  Liew's  Method  for  Blister  Copper.  —  This  is  the  standard 
method  for  copper  material.  Weigh  out  duplicate  samples  of 
i  assay  ton  each  of  copper  borings,  add  350  c.c.  cold  water  and 
100  c.c.  HNO3  (sp.  gr.  1.42),  and  set  in  a  cool  place  for  20  hours, 
stirring  from  time  to  time.  Then,  if  the  copper  is  not  dissolved, 
add  from  5  to  30  c.c.  more  of  concentrated  acid.  At  the  end  of 
26  to  28  hours  the  solution  of  the  copper  is  complete.  Do  not 
apply  heat  in  order  to  minimize  as  much  as  possible  the  solution 
of  small  quantities  of  gold,  by  whatever  action  this  may  take 
place.  The  oxides  of  nitrogen  in  the  solution  are  removed  by 
blowing  air  into  it  for  20  to  30  minutes. 

Salt  solution  (containing  5.4207  grams  of  NaCl  per  1000  c.c.) 
is  added  in  sufficient  quantity  to  precipitate  the  Ag  present  as 
chloride,  i  c.c.  of  this  solution  will  precipitate  i  mg.  of  Ag, 
and  an  excess  of  4  to  8  c.c.  above  that  required  for  the  Ag  should 
be  added.  If  the  amount  of  Ag  in  the  copper  is  small,  add  10  c.c. 
of  a  saturated  solution  of  lead  acetate  and  2  c.c.  of  concentrated 
H2SO4  in  order  to  form  PbSO4  to  aid  in  settling  the  silver  chloride. 
Let  this  stand  for  about  12  hours  and  filter  the  precipitate  into 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES        99 

the  proper  sized  filter,  and  wash  it  well  into  the  point  of  the  filter 
paper.  Dry  the  filter  carefully  in  the  air  bath,  and  when  dry, 
add  8  grams  of  test  lead  on  top  of  the  precipitate,  and  carefully 
transfer  to  a  scorifier  containing  2  grams  of  lead.  This  is  placed 
in  the  muffle,  heated  just  to  incipient  redness,  and  the  filter 
papers  burnt  off,  but  only  until  the  flame  disappears,  and  not 
into  ash.  This  takes  only  a  minute  or  so,  the  precaution  being 
taken  to  prevent  loss  of  silver  by  volatilization  as  AgCl,  the  lead 
and  carbon  present  reducing  the  AgCl  to  Ag.  Then  add  3  to 
4  grams  of  PbO,  and  the  same  amount  of  borax  glass,  raise  the 
heat  until  well  molten,  and  pour.  No  scorification  is  necessary, 
as  no  impurities  are  present.  The  lead  button  will  weigh  5  to 
8  grams  and  is  cupeled  with  feather  litharge.  The  results  should 
check  within  0.2  to  0.3  oz.  for  Ag  and  very  closely  for  gold.1 

Combination  Assay  for  Matte.  —  The  above  method  of  treating 
in  the  cold  is  rarely  suitable  for  mattes,  as  heat  is  usually  essential 
in  order  to  insure  a  decomposition  of  the  matte  in  a  reasonable 
length  of  time.  Take  2  duplicates  of  i  assay  ton  each  and  treat 
in  large  beakers,  provided  with  watch-glass  covers,  with  100  c.c. 
of  distilled  water  and  50  c.c.  HNO3  (sp.  gr.  1.42).  After  the 
violent  chemical  action  subsides,  add  50  c.c.  more  of  concentrated 
acid,  and  warm  the  beakers  on  a  hot-plate  until  everything 
soluble  is  dissolved:  usually  the  residue  is  white  or  grayish. 
Next  evaporate  a  considerable  part  of  the  acid  by  boiling,  ex- 
pelling all  of  the  nitrous  fumes,  dilute  to  500  c.c.,  add  3  c.c.  of 
concentrated  H2SO4,  10  c.c.  of  saturated  lead  acetate  solution, 
and  enough  salt  solution  of  the  strength  mentioned  for  blister 
copper  to  precipitate  the  silver;  then  stir  briskly  and  let  them 
stand  over  night.  Next  morning  warm  the  solutions  on  a  steam 
bath  and  filter  through  rather  thick  filter-paper. 

Filtrates  must  be  perfectly  clear  and  free  from  suspended 
PbSO4.  Wash  beakers  and  residue  thoroughly  with  hot  water, 
dry  the  filters  in  an  air  bath,  and  then  wrap  them  up  in  about 
8  grams  of  sheet  lead  and  scorify  with  40  grams  of  test  lead  and 
i  gram  of  borax  glass.  Cupel  the  buttons  with  feather  litharge. 
Re-assay  the  slag  from  the  scorification  and  the  cupel  and  add 
the  resultant  gold  and  silver  to  the  assay. 

1  R.  W.  Van  Liew,  in  "Eng.  and  Min.  Journ.,"  LXIX,  pp.  498  et  seq. 
2"  Assay  of  Copper  and  Copper  Matte,"  in  Trans.  A.  I.  M.  E.,  XXV. 


ioo  A   MANUAL  OF   FIRE   ASSAYING 

When  heavy  copper  ores  are  to  be  assayed  by  this  method, 
which  are  apt  to  leave  large  amounts  of  silicious  residue,  the 
general  method  for  mattes  is  followed,  except  that  the  residues 
after  filtering  and  drying  are  treated  as  follows: 

Take  a  2o-gram  crucible  and  place  in  it  i  assay  ton  of  PbO; 
then  put  the  filter-paper  containing  the  residue  on  top  of  this, 
place  the  crucible  in  the  mouth  of  the  muffle  at  a  low  heat,  burn 
off  the  filter-paper  until  the  flame  subsides,  remove  from  the 
muffle,  put  a  cover  on  the  crucible,  and  allow  to  cool.  When 
cold  add  0.5  assay  ton  PbO,  15  grams  of  Na2CO3,  2  grams  of 
argol,  mix  well  with  a  spatula,  and  put  on  a  cover  of  borax  glass. 
Then  proceed  as  in  the  ordinary  assay. 

General  Precautions  to  be  Observed  in  the  Combination  Assay.  - 
The  combination  methods  on  copper  material  agree  well  with  the 
standard  scorification  methods  for  the  same  material  when  cor- 
rection for  cupel  loss  is  made  for  the  latter  method.  The  scori- 
fication methods  will  often  seem  to  give  higher  results,  but  this 
is  in  most  cases  due  to  the  fact  that  the  silver  beads  frequently 
contain  from  2.5  to  4  per  cent,  copper.  The  combination  method 
gives  in  most  cases  (Van  Liew's  method  possibly  excepted) 
uniformly  lower  results  in  gold  (4  per  cent.)  than  the  standard 
corrected  scorification  method.  This  is  generally  ascribed  to  the 
formation  of  nitrous  acid  (HNO2)  during  solution,  which,  in 
connection  with  nitric  acid,  is  said  to  have  a  solvent  action  on 
gold;  but  such  authorities  as  W.  F.  Hillebrand  1  dispute  this. 
The  solution  may  be  due  to  the  formation  of  H2SO4  during  solu- 
tion, as  the  mixture  of  this  acid  and  HNO3  has  a  solvent  action, 
or  to  the  presence  of  impurities  like  chlorides  or  HC1,  etc.,  or 
possibly  to  the  presence  of  nitrates,  particularly  those  of  iron  or 
copper.  The  fact  that  the  combination  method  on  copper- 
bearing  material  gives  low  results  is,  however,  well  established. 

Owing  to  the  number  of  manipulations  in  the  combination 
assay,  it  is  often  apt  to  give  low  results  in  the  hands  of  inexpe- 
rienced chemists,  mainly  due  to  the  mechanical  losses  in  handling. 
The  directions  given  should  be  carefully  followed,  especially  those 
regarding  amount  of  solution,  strength  of  acid,  temperature, 
time,  etc.  Neatness  is  indispensable.  The  HNO3  must  be  pure. 

1  W.  F.  Hillebrand  and  E.  T.  Allen,  "Comparison  of  Wet  and  Crucible  Fire 
Methods  for  Gold  Telluride  Ores,"  Bull.  253,  U.  S.  G.  Survey. 


THE   ASSAY   OF  ORES  CONTAINING    IMPURITIES       101 

The  directions  regarding  the  burning  off  of  the  filter-paper  must 
be  closely  followed.  The  amount  and  strength  of  the  salt  solu- 
tion must  be  carefully  adhered  to  and  it  must  be  added  at  the 
proper  time.  Some  assayers,  instead  of  adding  salt  solution  at 
the  same  time  as  H2SO4  and  PbC2H3O2,  filter  off  the  residue 
containing  the  gold  and  make  a  separate  precipitation  for  the 
silver,  believing  that  the  addition  of  a  salt  solution  may  cause  a 
slight  redissolving  of  the  gold.  At  this  point  of  the  assay  that 
is,  however,  hardly  probable.  A  large  amount  of  NaCl  is  to  be 
avoided,  as  AgCl  is  very  appreciably  soluble  in  brine.  C.  White- 
head  recommends  NaBr  instead  of  NaCl  for  this  reason. 

Combination  Method  for  Precipitates  from  the  Cyanide  Process.1 
Where  the  troublesome  base-metal  impurity  is  zinc  instead 
of  copper,  as  in  this  case,  sulphuric  acid  can  be  substituted  with 
advantage  for  HNO3.  The  method  is  as  follows: 

Of  the  precipitates  o.io  assay  ton  is  taken,  placed  in  a  beaker, 
and  20  c.c.  of  sulphuric  acid  (concentrated)  and  60  c.c.  of  water 
are  added.  This  is  heated  on  a  hot  plate  for  about  one  hour,  or 
until  zinc  and  zinc  oxide  are  in  complete  solution.  Add  salt 
solution  of  the  strength  already  mentioned  in  the  paragraph  on 
Van  Liew's  method  for  blister  copper,  in  slight  excess,  to  pre- 
cipitate the  silver  present,  remembering  that  i  c.c.  will  precipitate 
I  mg.  of  silver.  Stir  briskly  with  glass  rod  to  agglomerate  the 
silver-chloride  formed. 

The  residues  are  then  filtered  through  the  proper  sized  filter, 
carefully  washed  with  hot  water  into  the  point  of  the  filter-paper, 
and  dried  in  the  air  bath  at  a  low  heat.  After  drying,  transfer 
to  a  2o-gram  crucible  containing  i  assay  ton  of  litharge,  and  burn 
the  filter-paper  off  in  the  manner  already  described.  Then  add 
15  grams  of  soda  and  2  grams  of  argol,  mix  thoroughly,  and 
cover  with  a  heavy  cover  of  borax  glass.  Fuse  and  cupel  the 
resultant  lead  button.  Weigh  the  gold  and  silver  bead,  and 
from  a  preliminary  assay  determine  the  proper  amount  of  silver 
necessary  in  order  to  inquart  the  bead.  The  amount  of  silver 
should  be  just  about  2.5  times  the  amount  of  gold.  Roll  out  the 
bead,  after  flattening  with  a  hammer,  until,  after  repeated 
rollings,  the  fillet  will  have  about  the  thickness  of  a  visiting  card. 

1  Fulton  and  Crawford,  "Notes  on  Assay  of  Zinc  Precipitates  Obtained  in 
the  Cyanide  Process,"  in  "Sch.  Mines  Quart.,"  XXII,  p.  153. 


102  A  MANUAL  OF   FIRE  ASSAYING 

It  is  best  to  anneal  the  bead  at  a  red  heat  between  the  various 
rollings,  in  order -to  prevent  cracking  on  the  edges.  Then  part 
in  a  parting  flask  in  hot  nitric  acid  having  a  specific  gravity  of 
1.26.  Boil  twice  for  at  least  20  minutes  each  time,  in  order  to 
insure  the  complete  removal  of  the  silver.  This  method  of 
parting  leaves  the  gold  in  one  coherent  mass,  termed  a  "cornet," 
and  is  identical  with  the  method  practised  in  the  gold  bullion 
assay. 


SPECIAL  METHODS  OF  ASSAY 

Telluride  Ores.  —  Gold  ores  carrying  the  values  in  the  form 
of  tellurides  of  gold  and  silver,  mainly  calaverite  and  sylvanite, 
are  more  difficult  of  assay  than  ordinary  gold  ores,  and  special 
methods  are  essential  in  order  to  get  good  results.  The  scorifica- 
tion  assay  is  not  reliable  for  telluride  ores,  giving  almost  uniformly 
low  results.  It  is  not  used  by  assayers  and  chemists  of  the  great 
telluride  ore  district  in  Colorado  —  Cripple  Creek.  It  seems  that 
in  scorification  the  main  cause  of  loss  is  volatilization,  for  while 
the  slag  loss  is  higher  than  for  ordinary  ores,  slag  and  cupel  cor- 
rections still  leave  the  results  from  this  assay  far  below  those  of 
the  crucible  assay  when  properly  performed. 

Tellurium  is  very  tenacious  in  its  hold  on  gold  and  silver,  and 
if  a  high-grade  telluride  ore  be  assayed,  even  by  special  method, 
the  beads  from  the  cupellation  will  frequently  still  contain  tellu- 
rium.1 In  the  crucible  assay  the  losses,  which  are  somewhat 
greater  than  in  ordinary  ores,  occur  in  the  slag,  and  from  the 
presence  of  the  Te  in  the  lead  button,  causing  absorption  of 
values  by  the  cupel.  The  aim  in  the  crucible  assay  is  to  remove 
the  tellurium  from  the  gold  and  silver  and  slag  it.  This  is  best 
accomplished  by  the  presence  of  considerable  litharge,  and 
otherwise  properly  balancing  the  flux.  The  flux  recommended 
quite  generally  by  Cripple  Creek  assayers  is  made  up  as  follows: 

Potassium  carbonate ....  7      parts  Flour i .  o  parts 

Sodium  carbonate 6      parts  Litharge 30-0  parts 

Borax  glass 5.5  parts 

This  is  for  the  ordinary  silicious  Cripple  Creek  ores.  About 
75  grams  of  this  flux  is  used  with  0.5  assay  ton  of  ore.  This  gives 
the  following  charge: 

1  E.  C.  Woodward,  "Cupel  Losses  in  Telluride  Ores,"  in  "West.  Chem.  and 
Met.,"  I,  p.  120. 

103 


104  A   MANUAL  OF   FIRE  ASSAYING 

Ore 0.5  assay  ton         Borax  glass 8.5  grams 

PbO  45 . 5  grams  Na2CO3   9.0  grams 

Flour    1.5  grams  K2COa   10. 5  grams 

The  heat  recommended  is  such  that  a  temperature  of  1084°  C, 
the  melting-point  of  gold,  is  reached  at  the  mouth  of  the  muffle. 
Some  assayers  recommend  a  somewhat  greater  temperature  to 
insure  the  decomposition  of  the  tellurides.  The  time  of  fusion 
should  be  about  45  to  50  minutes.  It  is  essential  to  recognize 
that  the  flux  recommended  above  for  tellurides  does  not  make 
what  can  be  strictly  termed  an  "excess-litharge  charge." 

Hillebrand  and  Allen  1  recommend  the  following  charge  for 
Cripple  Creek  ores: 

Ore i  assay  ton         Borax  glass 10  grams 

NaHCOs i  assay  ton         Reducing  agent  (if  necessary) 

PbO  6  assay  tons       Salt  cover 

This  approaches  more  nearly  the  excess-litharge  charge. 

It  is  essential  in  telluride  ores  to  have  the  sample  crushed  to 
1 20-  or,  better,  to  i5O-mesh.  The  reasons  for  this  is  that,  owing 
to  the  irregular  distribution  of  values  in  these  ores,  fine  crushing 
is  required  to  get  a  true  sample,  and  also  because  the  low  melting- 
point  of  the  charge  usually  employed  makes  this  essential. 

The  precise  behavior  of  tellurium  in  the  crucible  assay  has  not 
been  completely  investigated.  It  is  probable  that  the  greater 
part  of  it  is  oxidized  to  a  tellurate,  probably  that  of  soda,  under 
the  oxidizing  conditions  which  exist  in  the  usual  telluride  ore 
charge.  It  is  stated  2  that  in  the  oxidizing  roasting  of  Cripple 
Creek  telluride  ores,  in  their  preparation  for  chlorination  or 
cyanidation,  the  greater  part  of  the  tellurium  in  the  raw  ore  is 
found  in  the  roasted  ore  as  a  tellurite  of  iron.  Some  assayers 
add  an  iron  nail  to  the  assay,  not  so  much  to  desulphurize  as  to 
provide  an  excess  of  iron  for  the  purpose  of  combining  the  tel- 
lurium with  it,  as  in  the  case  of  sulphur. 

For  the  quantity  of  tellurium  present,  its  influence  on  the 
assay  is  certainly  profound.  The  following  table  gives  an  idea 
of  the  quantity  present: 

1  "A  Comparison  of  a  Wet  and  Crucible  Fire  Methods  for  the  Assay  of  Gold 
Telluride  Ores,"  Bull.  No.  253,  U.  S.  G.  Survey. 

2  Trans.  I.  M.  M.,  Ill,  pp.  49,  50. 


SPECIAL  METHODS  OF  ASSAY  105 

TABLE  XVIII.  —  QUANTITY   OF  TELLURIUM  IN    ORES 


ELEMENT 

CRIPPLE  CREEK 
ORE 

CRIPPLE  CREEK 
ORE 

BLACK  HILLS 
CAMBRIAN 

BLACK  HILLS 
CAMBRIAN 

Tellurium 
Gold  
Silver   .... 

0.0742  per  cent. 
0.0506  per  cent. 
0.0075  Per  cent. 

0.092    per  cent. 
0.060    per  cent. 
0.0103  per  cent. 

0.0033  Per  cent. 
0.0026  per  cent. 

o.oio  per  cent. 
0.003  Per  cent. 

As  already  stated,  tellurium  is  with  difficulty  separated  from 
gold  and  silver,  and  in  spite  of  an  oxidizing  charge  is  frequently 
carried  down  in  the  lead  button.  The  loss  then  takes  place  in 
the  cupel,  tellurium  causing  a  heavy  absorption.  Some  loss, 
however,  takes  place  by  volatilization.  There  is  also  a  somewhat 
higher  slag  loss  in  the  telluride  assay  than  in  the  assay  of  ordinary 
ores.1  Hillebrand  and  Allen,  already  quoted,  assayed  telluride 
ores  by  the  combination  wet-and-dry  assay,  getting  the  gold  and 
silver  free  from  tellurium,  but  found  that  the  crucible  assay  as 
ordinarily  performed  for  telluride  ores  gave  just  as  satisfactory, 
if  not  better,  results. 

A  Study  of  the  Assay  of  Black  Hills  Cambrian  Ores.  —  These 
ores  are  probably  complex  tellurides.  The  ores  were  oxidized 
and  of  the  following  average  composition: 

SiO2  =  71.5  per  cent.;  Fe2O3  =  16.3  per  cent.;  A12O3  =  4.8  per 
cent.;  CaO  =  1.5  per  cent.;  Gold  =  0.79  oz. ;  Ag  =  o.  10  oz. 

Samples  of  this  type  of  ore,  representing  controls  on  car-load 
lots,  were  assayed  by  assayers  A  and  B  in  the  same  laboratory, 
with  the  same  kind  of  cupels,  and  great  regard  to  temperature 
of  cupellation.  Assayer  A  made  fusions  on  i  assay  ton  lots,  in 
triplicate,  with  the  following  stock  flux: 

Na2COs   3.25  parts         Borax  glass 5.00  parts 

K2COa 2.25  parts          Argol    i.oo  parts 

PbO   18.00  parts  29.50  parts 

The  amount  of  flux  used  was  4  assay  tons  per  assay  ton  of 
ore,  with  quite  a  heavy  borax  glass  cover.  Fusions  made  at 
1 1 00°  C,  approximately. 

1  C.  H.  Fulton,  "Sen.  Mines  Quart.,"  XIX.  F.  C.  Smith,  Trans.  I.  M.  M., 
IX,  p.  344.  "Min.  Rep."  51,  p.  163.  Hillebrand  and  Allen,  Bull.  No.  253,  U.  S. 
G.  Survey,  pp.  12,  14. 


106  A   MANUAL  OF   FIRE   ASSAYING 

The  stock  flux  is  equivalent  to  the  following  charge: 

Ore i      assay  ton          PbO  73.2  grams 

Na2CO3    13.2  grams  Borax  glass 20.3  grams 

K2COs   9.1  grams  Argol 1    4.0  grams 

On  account  of  the  negligible  quantity  of  Ag  present,  every 
assay  was  salted  with  Ag.  The  beads  were  parted  in  acid  i  to  9, 
and  were  in  each  case  required  to  check  against  each  other  in 
weight.  The  beads  were  then  weighed  together  and  the  resultant 
weight  divided  by  3  to  obtain  the  amount  of  gold. 

Assayer  B  made  assays  on  the  same  pulp  samples  with  the 
following  stock  flux: 

Na2COs   3.25  parts         Borax  glass 2.00  parts 

K2CO3    2.25  parts         Argol    0.75  to  i.oo  part 

PbO   22.00  parts  30-25  parts 

Three  assay  tons  of  flux  were  used  to  each  0.5  assay  ton  of 
ore,  with  a  soda  cover  one-quarter  inch  thick.  Assays  were  made 
in  quadruple,  all  fusions  being  salted  with  Ag,  parted  in  i  to  4 
acid,  and  the  beads  required  to  check  against  each  other  in  weight 
and  then  weighed  together,  and  the  sum  divided  by  2  to  get  the 
value  per  ton. 

The  stock  flux  is  equivalent  to  the  following  charge: 

Ore 0.5  assay  ton        PbO  67    grams 

Na2CO3   9     grams  Borax  glass 6     grams 

K2COs   6     grams  Argol    2.5  grams 

The  results  of  these  series  of  assays  were  as  follows: 

LOT  No.  ASSAYER  A  ASSAYER  B 

Oz.  Au  per  ton  Oz.  Au  per  ton 

88870 0.74 0.78 

88832  0.80 0.83 

88874 0.71 0.80 

88823 .t 0.82 0.98 

88721 0.80 0.88 

88851 0.85 0.89 

88818 0.85 0.91 

88940 0.79 0.84 

3669 0.82 0.9 1 

88853 °-85 o-9i 

0.81 0.83 


1  This   amount  of  argol   required   because  ores  are  oxidizing.     The  button 
produced  usually  22  to  25  grams. 


SPECIAL  METHODS  OF  ASSAY 


107 


LOT  No.  ASSAYER  A  ASSAYER  B 

Oz.  Au  per  ton  Oz.  Au  per  ton 

71957 °-79 0.82 

88826 0.77 0.82 

3843 °-77 0.81 

88780 0.80  0.88 

22522 0.66 0.73 

98509 0.69 0.79 

22050 :  .0.50 0.58 

Assayers  A  and  B  then  exchanged  fluxes,  and  as  they  checked 
each  other's  previous  results  closely,  it  became  evident  that  the 
flux  of  assayer  A  was  ill-balanced  and  would  not  give  good  results. 
Slag  and  cupel  corrections  were  made  by  Assayer  A  on  assays 
made  with  his  flux,  but  even  these  corrections  added  failed  to 
bring  his  results  up  to  those  of  assayer  B. 

The  question  arises  as  to  what  is  the  specific  trouble  with 
flux  A.  On  examination,  it  will  be  found  to  contain  an  excessive 
amount  of  borax  glass,  especially  when  the  cover  is  considered. 
It  is  very  probable  that  the  acidity  of  the  charge  (although  a  good 
fluid  slag  is  obtained)  is  so  great,  taking  into  account  both  the 
silica  of  the  ore  and  the  borax  glass,  that  the  ore  is  not  com- 
pletely decomposed  by  the  basic  ingredients  of  the  charge;  i.e., 
the  soda  and  litharge  become  saturated  with  borax  and  then  do 
not  completely  decompose  the  silicious  ore.  The  fact  that  re- 
assays  of  the  slag  do  not  bring  the  results  up  to  the  figures  obtained 
by  assayer  B  does  not  necessarily  imply  that  the  slag  does  not 
contain  these  values,  as  the  charge  used  to  flux  the  slags  and 
cupels  again  contains  much  borax  glass,  so  that  practically  the 
same  conditions  obtained  as  before. 

The  Assay  of  Copper- Bearing  Material.  —  Copper-bearing  ma- 
terial includes  ores  containing  copper  and  furnace  products, 
chiefly  mattes,  blister  copper,  etc.  Copper,  which  in  the  assay 
has  a  strong  tendency  to  go  into  the  lead  button,  causes,  when 
present  in  sufficient  quantity,  serious  losses  by  cupel  absorption. 
Therefore  all  methods  of  assay  for  this  class  of  material  endeavor 
to  eliminate  copper  from  the  lead  button  to  be  cupeled.  A 
standard  method  for  the  assay  of  material  high  in  copper,  espe- 
cially for  Ag,  is  the  combination  assay  for  blister  copper  and 
mattes,  described  in  Chapter  IX. 

Another  standard  method,  especially  for  gold,  and  one  that 


io8  A   MANUAL  OF   FIRE  ASSAYING 

is  carried  out  frequently  as  a  check  to  the  above,  is  the  scorifica- 
tion  method.  This  is  performed  as  follows: 

Ten  samples,  of  o.  10  assay  ton  each,  are  taken  and  placed  in 
3-inch  scorifiers  with  50  grams  of  test  lead  (the  silver  content  of 
which  is  accurately  known) ;  25  grams  of  the  lead  are  mixed  with 
the  matte,  or  borings,  etc.,  and  the  other  25  grams  used  as  a 
cover.  On  top  of  the  charge  is  placed  i  gram  each  of  silica  and 
borax  glass.  The  scorification  is  carried  on  at  a  moderate  tem- 
perature until  the  assays  are  just  about  to  slag  over,  which  takes 
usually  about  25  minutes,  and  then  they  are  poured.  The  resul- 
tant button  will  weigh  about  15  to  16  grams  and  be  quite  hard 
with  copper.  The  buttons,  cleaned  from  slag,  are  scorified,  test 
lead  being  added  to  make  the  total  lead  up  to  40  grams.  The 
second  scorification  will  take  about  30  minutes  and  the  resultant 
buttons  will  weigh  from  10  to  12  grams.  These  are  cupeled  in 
10  separate  cupels,  placed  so  as  to  be  subject  to  uniform  tem- 
perature, i.e.,  in  one  horizontal  row  across  the  muffle.  Cupellation 
temperature  should  not  exceed  750°  C.  (a  full  cherry-red)  and 
should  be  finished  with  a  hotter  blick. 

The  beads  are  weighed  separately  and  then  together.  They 
are  then  grouped  in  two  lots  of  5  each,  which  are  parted  in  acid, 
strength  i  to  9,  the  beads  being  kept  in  this  acid  at  nearly  boiling 
temperature  for  20  minutes  and  finished  for  5  minutes  with 
[.42  sp.  gr.  acid  (full  strength).  The  ten  cupels  are  taken  in  lots 
of  two  each  (only  the  litharge-stained  part  is  taken),  crushed  to 
pass  loo-mesh  and  assayed  by  the  following  charge: 

100  grams  PbO  45  grams  borax  glass 

20  grams  Na2CO3  3  grams  argol 

Soda  cover 

The  lead  buttons  are  cupeled,  and  the  silver  and  gold  obtained 
added  to  the  first  weights.  The  scorification  slags  may  also  be 
reassayed  and  this  correction  added,  but  in  practice  the  cupel 
correction  is  the  only  one  usually  allowed.  Sometimes  no  correc- 
tion is  allowed.  It  is  to  be  noted  that,  even  with  a  rescorifica- 
tion  of  the  first  button  of  the  assay,  the  final  silver  beads,  from 
55  per  cent.  Cu  matte  containing  180  oz.  Ag  per  ton  and  2.31  oz. 
gold,  will  contain  from  2.5  to  4  per  cent,  copper,  which  must  be 
deducted  in  order  to  get  correct  silver  results.  (For  a  further 


SPECIAL  METHODS  OF  ASSAY  109 

discussion  of  scorification  slag  losses  and  cupel  absorption  in 
assaying  copper-bearing  material,  see  Chapter  XI.) 

The  scorification  method  is  generally  employed  for  the  deter- 
mination of  gold  in  mattes,  and  the  combination  method  for  the 
determination  of  silver.  Of  recent  years,  special  crucible  methods 
for  copper  mattes  and  copper-bearing  material  have  been  devel- 
oped with  considerable  success.1 

A  satisfactory  method  on  copper  mattes,  up  to  20  per  cent. 
copper  and  high  in  gold  and  silver,  is  practised  by  the  Standard 
Smelting  Company,  at  Rapid  City,  S.  Dak.  The  matte  sample 
is  put  through  a  i2O-mesh  screen,  and  for  controls  4  assays  of 
o%25  assay  ton  each  are  made,  with  the  following  stock  flux: 

Silica    .................  ii  parts         Sodium  carbonate    .....  25  parts 

Litharge  ...............  70  parts         Niter  ........  ..........    5  parts 

An  0.25  assay  ton  matte  is  run  with  a  3.5  assay  ton  flux  and 
a  thin  borax  glass  cover.  The  flux  figured  to  the  charge  is  as 
follows  : 


0.25  assay  ton  ...........  matte         24.0  grams  ............. 

10.5  grams  ...............  SiC>2  5.0  grams  .............  KNOs 

67.0  grams  ...............  PbO 

The  heat  used  is  high  and  the  fusion  short,  giving  a  clean 
fluid  slag  and  a  bright  button  of  approximately  20  grams.  These 
buttons  are  cupeled  directly  for  gold  and  silver.  One  cupel  and 
one  slag  are  then  re-run  in  the  same  crucible  that  the  original 
fusion  was  made  in,  and  the  result  of  the  four  corrections  added 
to  the  sum  of  the  original  buttons.  No  scorification  is  made 
before  cupellation.  The  average  correction,  on  the  usual  grade 
of  matte  (5  oz.  Au,  40  oz.  Ag),  is  2.5  per  cent,  gold  and  5.5  per 
cent,  silver.  Below  is  a  comparison  of  this  method  with  the 
standard  scorification  assay,  including  cupel  and  slag  correc- 
tion. The  copper  content  of  this  matte  was  19.98  per  cent. 

1  "An  All-Fire  Method  for  the  Assay  of  Gold  and  Silver  in  Blister  Copper," 
in  Trans.  A.  I.  M.  E.,  XXXII.  Perkins,  "The  Litharge  Process  for  the  Assay 
of  Copper  Bearing  Ores,"  ibid.,  XXXI,  p.  913. 


io  A   MANUAL  OF    FIRE   ASSAYING 

TABLE   XIX.  —  COMPARISON   OF   METHODS   IN  ASSAY 


CRUCIBLE  METHOD 

SCORIFICATION  METHOD 

GOLD  (oz.) 

SILVER  (oz.) 

GOLD  (oz.) 

SILVER  (oz.) 

Original  assay  

4.10 

0.  10 

36.24 

I  .00 

3.90 
0.25 

35  -°7 
i-95 

Correction     

4.20 

37-24 

4-i5 

37.02 

The  returns  of  this  pulp  by  the  refiner  were:  gold,  4.19  oz.; 
silver,  36.71  oz. 

Matte  No.  1545;  copper  content,  17.6  per  cent. 

TABLE   XX.  —  COMPARISON   OF  METHODS   IN   ASSAY 


CRUCIBLE  METHOD 

SCORIFICATION  METHOD 

GOLD  (oz.) 

SILVER  (oz.) 

GOLD  (oz.) 

SILVER  (oz.) 

Original  assay  
Correction   

3-42 
o.  io 
3-52 

3i-94 
1.85 

33-79 

3-40 

0.  II 

3-5i 

3i-86 
i-93 
33-79 

The  following  table  shows  results  by  this  method  with  correc- 
tion and  refiners'  results  (by  same  method  without  correction): 

LOT  No.  CRUCIBLE  METHOD: 

Standard  Smelting  Company 


i39i  

I  AO4 

Gold 

17-73 
•  •  I7-75 

Silver 

75-4 
72.  c 

IJ.I  2    • 

ii.  ?f 

4^.7 

1435  
i4.<\o 

10.02 
.     O.IO 

39.9 
44.6 

•T/I  C7 

6.80 

48.75 

I4.c8 

.    Q.CK 

58.37 

I4.7O 

.     8.235 

50.86 

14.71 

4.845 

4O.52 

14.84. 

.    7-34 

45.04 

I48o 

6.45 

47.08 

I  COO. 

c.78 

45.34 

ISO3.  . 

.   4.61 

27.11 

CRUCIBLE  METHOD  : 

Refiner 

Gold 

Silver 

Copper 

17.67 

74-13 

7-2 

17.625 

72.42 

9-5 

11.145 

42.95 

10.3 

10.065 

39.08 

12.9 

9.02 

43.01 

13.2 

6.815 

46.28 

15.02 

9-935 

52.31 

19.9 

8.24 

52.31 

20.5 

4-87 

3948 

18.4 

7.265 

44.68 

18.27 

6.83 

46.67 

19.4 

5.84 

45.06 

20.5 

4-58 

26.51 

18.8 

SPECIAL  METHODS  OF  ASSAY  „-, 

LOT  No.  CRUCIBLE  METHOD:  CRUCIBLE  METHOD  : 

Standard  Smelting  Company  Refiner 

Gold              Silver  Gold           Silver  Copper 

i5J3 3-8l5            32-75  3-8°              3J-55  "-8 

3-12               39-62  3-205             33.84  12.4 

3-10              33-°°  3-36              3!-i4  18.7 

4.20              37.24  4.19               36.71  20.0 


A  typical  sample  of  matte  on  which  these  assays  were  made 
analyzes  as  follows: 

Gold 4.10  oz.  per  ton  Silica    3.3  per  cent. 

Silver   3J-55  oz-  Per  ton  Lime 0.5  per  cent. 

Copper 17.4    per  cent.  Sulphur   29.1  per  cent. 

Iron    45.9    per  cent.  Lead trace 

Zinc    2.5     per  cent. 

The  crucible  charge  employed  can  readily  be  modified  to 
apply  to  mattes  higher  in  copper  or  greater  in  reducing  power. 

Perkins'  excess-litharge  method  has  already  been  described. 
He  states  that  for  low-grade  copper-bearing  material  (2  to  4  per 
cent.),  5  assay  tons  of  PbO  to  0.5  assay  ton  of  ore  will  remove 
most  of  the  copper,  if  the  balance  of  the  fluxes  are  properly 
proportioned,  i.e.,  if  they  have  ample  free  PbO  to  dissolve  copper 
oxides.  For  high-grade  mattes,  etc.  —  48  to  60  per  cent,  copper 
—  8  assay  tons  of  PbO  to  o.  i  assay  ton  of  matte  will  remove  most 
of  the  copper.  Perkins  also  developed  a  crucible  method  for 
metallic  copper,  as  follows: 

Weigh  out  0.25  assay  ton  of  copper  borings,  divide  it  into 
3  approximately  equal  parts,  and  place  in  2O-gram  crucibles.  In 
this  way  weigh  out  4  sets,  getting  12  assays.  Into  each  crucible 
put  800  mgs.  of  powdered  sulphur,  mix  thoroughly  with  the 
copper,  and  then  on  top  of  this  put  the  following  flux,  being 
careful  not  to  mix  the  flux  with  the  copper: 


0.25  assay  ton          PbO  8.0  assay  ton 

K2CO3   0.25  assay  ton          SiO2   0.5  assay  ton 

Salt  cover 

Place  the  crucibles  into  a  dark-red  muffle  and  gradually  raise 
the  temperature  for  45  minutes  to  a  yellow  heat.  The  tempera- 
ture regulation  is  important,  and  it  is  necessary  to  produce  a 
neutral  or  reducing  atmosphere  in  the  muffle  by  the  presence  of 
coal  or  coke.  The  buttons,  weighing  about  18  grams  each,  are 


ii2  A  MANUAL  OF   FIRE  ASSAYING 

put  together  in  lots  of  three,  representing  0.25  assay  ton,  and 
scorified  at  a  low  heat.  The  resultant  buttons  should  weigh 
5  to  6  grams.  Each  of  these  buttons  is  now  rescorified  with 
25  grams  of  lead  at  a  low  heat,  until  6-gram  buttons  are  obtained. 
These  are  cupeled  with  feathers.  This  method  is  stated  to  give 
results  on  gold  equal  to  the  all-scorification  method,  and  on  silver 
equal  to  the  combination  method. 

The  Assay  of  Zinciferous  Ores  and  Meiallurgic  Products  Con- 
taining Zinc.  —  Zinc  most  frequently  occurs  in  ores  as  the  sul- 
phide, sphalerite,  and,  in  certain  metallurgical  products,  as  the 
metal  (zinc  cyanide  precipitates).  Zinc  boils  at  940°  C,  and 
rapidly  volatilizes.  Zinc  oxide  volatilizes  slowly  at  the  melting- 
point  of  silver  (962°  C.),  and  rapidly  at  a  white  heat.  Zinc 
silicates  alone  are  difficultly  fusible,  but  are  readily  so  when 
mixed  with  borax  or  boric  acid  or  ferrous  silicate.1  The  presence 
of  zinc  in  material  to  be  assayed  calls  for  certain  precautions, 
and  in  general  the  assay  is  difficult.  Metallic  zinc  has  a  great 
affinity  for  gold  and  silver,  greater  than  lead,,  as  is  shown  by  the 
Parkes  process  for  the  desilverization  of  lead  bullion.  Under 
oxidizing  influences  2  the  formation  of  zinc  oxide  and  its  volatili- 
zation causes  losses  of  gold  and  silver.  That  this  loss  is  mechani- 
cal does  not  make  it  less  serious.  The  boiling-point  of  zinc  and 
the  volatilization  of  its  oxide  occur  at  temperatures  somewhat 
below  the  normal  for  ordinary  scorification,  and  it  is  these  general 
facts,  coupled  with  the  fact  that  the  zinc  oxide  formed  is  with 
difficulty  soluble  in  litharge,  that  make  accurate  assay-results 
hard  to  obtain,  especially  in  scorification.  Zinc  containing  gold 
and  silver  may  be  distilled  off  and  volatilized  with  very  little 
loss  of  gold  and  silver,  if  the  conditions  are  reducing.3 

Scorification  is  frequently  employed  for  zinciferous  ores, 
although  it  is  not  generally  satisfactory.  When  used,  it  is  best 
carried  out  in  a  way  similar  to  that  adopted  for  copper-bearing 
material,  using  from  0.05  to  o.  10  assay  ton  of  ore  with  from 
50  to  80  grams  of  test  lead,  2  grams  of  borax  glass,  and  i  gram 
of  silica,  the  last  being  essential  to  flux  the  zinc  oxide  formed. 

1  Rose,  "Refining  Gold  Bullion,  etc.  in   Oxygen   Gas,"  in    Trans.  I.  M.  M, 
April,  1905. 

2  Williams,  in  Journ.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  III,  p.  132. 

3  Rose,  ibid.,  and  references. 


SPECIAL  METHODS  OF  ASSAY  113 

Otherwise  insoluble  scoria  and  crusts  form  on  the  scorifier.  Slag 
and  cupel  corrections  are  generally  necessary  and  from  5  to  10 
assays  are  made,  the  results  being  averaged.  As  zinc  is  readily 
oxidized,  lead  buttons  contaminated  with  zinc  are  not  to  be  feared 
and  rescorification  is  rarely  necessary.  Among  the  most  im- 
portant zinciferous  material  presented  for  assay  are  the  zinc-gold 
precipitates  from  the  cyanide  process.  Scorification  is  not  de- 
sirable for  these.1  They  are  best  assayed  by  the  crucible  method 
or  by  one  of  the  combination  methods  already  described. 

Crucible  Method.  —  The  crucible  method  best  suited  for  unox- 
idized  zinc  ores  is  the  niter  method,  with  sufficient  silica  present 
to  form  at  least  the  monosilicate  with  zinc.  Borax  glass  and 
much  litharge  is  also  desirable.  On  a  practically  pure  sphalerite 
the  following  charge  will  give  good  fusions  at  temperatures  of 
about  1 1 00°  C. : 

Ore 0.5  assay  ton          SiO2    8  grams 

Na2CO3    15      grams  KNO3 22  grams 

PbO   150     grams  Heavy  borax  glass  cover.2 

This  charge  can  be  modified,  as  regards  niter  and  silica,  to 
suit  any  sphalerite  ore. 

A  good  crucible  charge  for  cyanide  precipitates,  containing 
up  to  50  per  cent,  zinc,  is: 

Precipitates o.i  assay  ton          SiO2    5  grams 

Na2COs    5      grams  Na2B4O7 2  grams 

PbO   70     grams  Flour    i  gram 

Light  borax  glass  cover 

Assay  of  Plumbago  Crucibles  for  Gold  and  Silver.  —  Graphite 
or  plumbago  crucibles  are  extensively  used  in  the  smelting  of 
cyanide  zinc  precipitates,  and  the  old  discarded  ones  are  usually 
sold  in  lots  to  some  smelter;  they  often  contain  considerable  gold 
and  silver.  These  pots  present  difficulty  in  assaying,  chiefly  on 
account  of  the  graphite  and  zinc  contained.  From  a  given 
weight  of  sample,  the  metallics  and  scales  are  separated  by 
passing  the  material  through  a  i5O-mesh  screen,  and  a  regular 

1  "Notes  on  the  Assay  of  Zinc  Precipitates,  etc.,"  in  "Sch.  Mines  Quart.," 
XXII,  p.  153. 

2  A  similar  charge  is  recommended  by  Lay,  for  complex  zinc-lead  concentrate; 
see  "Min.  Ind.,"  XIII,  p.  287. 


H4  A  MANUAL  OF   FIRE   ASSAYING 

scale  assay  is  made  as  outlined  at  the  end  of  this  chapter.     The 
pulp  is  assayed  as  follows:  l 

From  0.05  to  o.io  assay  ton  is  taken  and  mixed  with  a  little 
more  than  one-half  its  weight  of  niter  and  30  grams  of  litharge, 
placed  in  a  2.5-inch  scorifier,  covered  with  30  grams  of  litharge 
and  afterward  with  a  thin  cover  of  borax  glass,  placed  in  a  muffle, 
and  fused  finally  at  a  yellow  heat.  The  buttons  are  cupeled, 
weighed,  and  parted  as  usual.  Crucible  assays  may  also  be 
made  on  this  material  by  the  niter  excess-litharge  fusion,  with 
a  charge  as  follows: 

o.i  assay  ton,  graphite  5  grams,  Na2CO3 

70     grams  PbO  5    to    n    grams    KNOs    (according    to 

5  grams  SiO2  carbon  contents  of  pulp) 

Borax  glass  cover 

In  both  methods  it  is  essential  that  the  amount  of  pulp, 
usually,  should  not  exceed  o.  i  assay  ton,  the  carbon  giving 
difficulties  with  greater  amounts  than  this. 

The  Assay  of  Residues  from  Zinc  Distillation  (containing  con- 
siderable carbon)  for  Silver  and  Gold.2  --  From  o.  10  to  0.5  assay 
ton  of  the  powdered  residue  is  mixed  with  35  grams  of  niter  and 
10  grams  of  Na2O2  (sodium  peroxide),  and  dropped,  in  lots  of 
5  grams  each,  into  a  red-hot  crucible  which  can  be  readily  covered, 
and  the  oxidation  reactions  permitted  to  complete  themselves. 
The  flux  then  added  consists  of  70  grams  of  litharge,  10  grams 
of  borax  glass,  10  grams  silica,  2  grams  argol  and  a  light  borax 
glass  cover.  The  fusion  is  carried  out  at  a  yellow  heat  and  the 
buttons  cupeled  as  usual. 

The  Assay  of  Antimonial  Ores  for  Gold  and  Silver.  —  Gold- 
and  silver-bearing  antimonial  ores,  such  as  stibnite,  jamesonite, 
etc.,  are  usually  assayed  by  the  niter  method,3  in  the  presence  of 
considerable  soda  and  niter,  to  induce  the  formation  of  the  anti- 
moniate  of  soda.  A  preliminary  assay  to  determine  the  amount 
of  niter  is  essential.  The  following  charge  is  recommended  for 
nearly  pure  stibnite:4 

JA  modification  of  T.  L.  Carter's  method;  see  "Eng.  and  Min.  Journ.," 
LXVIII,  p.  155. 

2K.  Sander,  in  "Eng.  and  Min.  Journ.,"  LXXIII,  p.  380. 

3  William  Kitto,  "The  Assay  of  Antimonial  Gold  Ores,"  in  Trans.  I.  M.  M., 
1906,  Nov.  8  and  Dec.  13. 

4  Smith,  "The  Assay  of  Complex  Gold  Ores,"  in  Trans.  I.  M.  M.,  IX,  p.  332. 


SPECIAL  METHODS  OF  ASSAY  115 

Ore .  .  .     0.5  assay  ton          KNOs 18  grams 

PbO    120     grams  Borax  glass 6  grams 

Na2CO3 10     grams  SiO2   10  grams 

Salt  cover 

The  fusion  should  be  conducted  slowly  and  at  a  low  tempera- 
ture. The  button  will  usually  contain  very  little  antimony,  the 
cupel  not  showing  scoria  or  cracks.  If  it  does  contain  enough  to 
cause  losses  in  cupellation,  the  buttons  should  be  scorified. 
Smith  !  gives  the  following  charge  for  ore  containing  approxi- 
mately 75  per  cent,  stibnite.  The  niter,  etc.,  can  be  varied  for 
the  ore  as  the  gangue  increases; 

Ore i  assay  ton  Borax  glass 8  grams 

PbO  75  grams  KNO3 20  to  25  grams 

Na2COs   25  grams  Salt  cover. 

Another  method,  practically  as  good  as  the  niter  method,  is 
the  roasting  with  charcoal  or  coke-dust.2  The  sample  of  ore, 
usually  i  assay  ton,  is  mixed  with  approximately  its  own  volume 
of  coke-dust  or  coal-dust,  placed  in  a  5-in.  roasting  dish,  covered 
with  another  dish,  and  roasted  in  the  muffle  with  closed  door, 
at  a  temperature  not  exceeding  a  dark  cherry-red  (635°  C),  for 
about  35  to  40  minutes.  This  will  cause  the  volatilization  of 
95  to  96  per  cent,  of  the  antimony  as  sulphide  without  appreciable 
loss  of  gold.  The  roast  should  have  a  yellow  appearance  when 
finished,  and  can  be  fused  with  the  following  charge: 

Roasted  ore  SiO2    7  grams 

PbO   70  grams         Argol    2  grams 

Na2COs    20  grams         Borax  glass  cover 

This  method  gives  good  results  on  jamesonite  ores. 

Arsenical  ores  are  assayed  by  the  same  methods  as  the  anti- 
monial  ores;  also  by  the  iron-nail  method,  although  this  last  is 
not  generally  to  be  recommended.  The  subject  of  the  best 
method  of  assay  of  antimonial  and  arsenical  ores  still  lacks 
thorough  investigation.  The  chief  points  may  be  outlined  as 
follows: 

i.  In  the  roasting,  unless  great  care  is  taken  as  regards 
temperature,  mechanical  loss  of  gold  and  silver  takes  place, 

1  Smith,  Ibid. 

2  Sulman,  Trans.  I.  M.  M.,  IX,  p.  340. 


n6  A   MANUAL  OF   FIRE   ASSAYING 

owing  to  the  rapid  disengagement  of  the  arsenic  and  antimony 
oxides,  or  sulphides  of  these  metals.  Unless  the  roast  is  con- 
ducted at  a  low  heat  and  in  the  presence  of  considerable  carbon, 
arseniates  and  antimoniates  of  base  metals  or  silver  may  form, 
holding  values  which  later  on  are  not  completely  decomposed  in 
the  crucible,  owing  to  their  stability  at  a  high  temperature,  the 
result  being  appreciable  slag  losses. 

2.  In  the  niter  method,  the  presence  of  much  niter,  with  its 
powerful   oxidizing   effect,    may   also   induce   the   formation   of 
arseniates  and  antimoniates,  containing  silver  and  possibly  gold, 
which  will  remain  in  the  slag. 

3.  In  the  iron-nail  method,  unless  the  fluxes  are  carefully 
adjusted  and  the  temperature  kept  below  1 100°  C,  speiss  carrying 
values  is  very  apt  to  form  above  the  lead  button,  and  thus  neces- 
sitate a  re-assay,  or  a  treatment  of  this  speiss. 

The  Assay  of  Sulphides,  mainly  Pyrite,  but  containing  Small 
Amounts  of  Copper,  Zinc  Sulphides,  etc.  —  Where  gold  only  has 
to  be  determined  in  ores  of  this  character,  the  roasting  method 
is  satisfactory.  This,  however,  proves  unreliable  for  silver,  and 
in  many  cases  (as  at  Leadville)  the  silver  contents  of  these  sul- 
phides are  the  most  important.  The  best  method,  after  many 
trials,  was  found  to  be  the  niter  fusion  on  comparatively  small 
lots  of  ore.  The  ore  has  the  following  analyses: 

Iron    33  to  44  per  cent.       Zinc    4  to  8      per  cent. 

Sulphur    38  to  45  per  cent.       Copper 0.5  to  3.5  per  cent. 

Insoluble 4  to  20  per  cent.       Lead o     to  0.4  per  cent. 

Four  assays  are  made  on  0.25  assay  ton  each,  with  3  to  4 
assay  tons  of  the  following  flux,  the  amount  depending  on  the 
reducing  power;  i.e.,  on  the  amount  of  sulphides  present: 

PbO 8     parts         SiO2   i  .5  parts 

KNOs i  .5  parts          Borax  glass i  .5  parts 

Na2CO3   3.0  parts 

Either  a  salt  or  a  soda  cover  is  used.  The  temperature  of 
fusion  is  brought  up  gradually  to  a  yellow  heat.  With  4  assay 
tons  this  gives  the  following  charge  *: 

1  See  also  W.  G.  Vail,  "Niter  Assay  for  Sulphide  Ores,"  in  "West.  Chem. 
and  Met.,"  II,  p.  14. 


SPECIAL  METHODS  OF  ASSAY  117 

Ore 0.25  .assay  ton          Na2COs   24  grams 

PbO   62        grams  Borax  glass u  grams 

KNO3 12       grams  SiO2   n  grams 

The  buttons  are  usually  clean,  and  separate  well  from  the  slag. 

Another  method  which  may  be  used  on  this  type  of  ore  is  the 
niter-iron  method.  This  has  the  advantage  that  no  preliminary 
assay  is  necessary  to  determine  the  amount  of  niter  for  the  proper 
size  button,  but  that  only  sufficient  niter  is  added  to  partially 
oxidize  the  sulphides,  the  iron  nails  being  relied  upon  to  decom- 
pose the  balance  of  the  ore.  On  ores  of  the  class  shown  by  the 
analysis,  the  following  charge  is  successful : 

Ore 0.5  assay  ton  SiO2   8  grams 

NaoCOs    25    grams  Borax  glass 8  grams 

PbO   30     grams  Iron  nails 2  to  3  tenpenny 

KNOa 15     grams  Thin  borax  glass  cover 

If  the  ore  has  a  lesser  reducing  power  than  shown  by  the 
analysis  given,  niter  and  silica  should  be  decreased  in  the  charge. 

The  Assay  of  Material  Containing  Metallic  Scales.  —  Ores  of 
this  kind  are  difficult  to  assay  and  obtain  correct  results,  as  the 
metallic  particles  (usually  gold)  are  so  unevenly  distributed  as  to 
make  it  practically  impossible  to  obtain  an  accurate  sample. 
Two  methods  of  assay  are  available: 

(a)  Approximately  500  grams  of  ore  (or  less,  if  deemed  advis- 
able) are  weighed  out,  crushed,  and  put  through  a  150-  or  200- 
mesh  screen,  care  being  taken  to  separate  out  the  scales  as  closely 
as  possible.     Screening  and  crushing  should. frequently  succeed 
each  other.     When  all  the  scales  have  been  separated  out,  they 
are  transferred  to  a  parting  cup  and  dissolved  in  3  to  5  c.c.  of 
nitro-hydrochloric  acid,  if  gold,  or  in  nitric  acid  if  silver  or  copper. 
The  pulp  is  then  heaped  up  into  a  cone  in  a  large  porcelain  dish, 
the  gold,  etc.,  solution  poured  on  the  apex  of  the  cone,  and  the 
parting  cup  washed  out  thoroughly  with  warm  distilled  water, 
using  no  more  than  is  necessary  to  completely  wash  it  out.     The 
bed  of  pulp  should  be  thick  enough  to  readily  absorb  all  of  the 
solution,  and  not  permit  it  to  penetrate  to  the  dish.     The  pulp  is 
then  dried  in  an  air  bath  at  120°  C,  thoroughly  mixed  on  glazed 
paper,  and  put  through  the  screen  repeatedly.     It  is  then  assayed 
by  the  method  suitable  to  it,  like  any  other  ore. 

(b)  From  200  to  500  grams  of  ore  are  weighed  out,  crushed 


n8  A   MANUAL  OF   FIRE  ASSAYING 

and  screened,  and  the  scales  separated,  as  described  above.  The 
scales  and  the  pulp  are  then  weighed  and  the  loss  in  dusting 
noted.  The  scales  are  assayed  by  scorification ;  the  lead  button 
is  cupeled,  and  the  bead  weighed  and  parted.  Then  15  grams 
of  the  ore  is  weighed  out  in  duplicate,  fused  with  the  proper 
charge,  the  lead  buttons  from  these  fusions  cupeled,  and  the 
beads  weighed  and  parted.  From  the  results  obtained,  the  total 
amount  of  gold  and  silver  in  the  original  ore  is  calculated,  con- 
sidering both  pulp  and  scales.  The  gold  and  silver,  respectively, 
found  is  multiplied  by  29.166  and  divided  by  the  original  weight 
of  ore,  taken  in  grams;  this  gives  the  value  in  ounces  per  ton. 

The  Assay  of  Ores  Containing  their  Chief  Value  in  Free  Gold.  - 
As  already  pointed  out,  these  ores  are  difficult  to  get  correct 
results  from.  Even  though  the  free  gold  particles  are  very  fine, 
it  is  impossible  to  distribute  them  uniformly  throughout  the  bulk 
of  the  sample.  The  only  proper  way  to  assay  material  of  this 
kind  is  to  take  from  3  to  5  Ibs.  of  the  ore,  crushed  through  a 
loo-mesh  screen,  place  in  a  large  jar  with  a  tight  screw  cover, 
mix  to  a  thick  pulp  with  water,  add  i  to  2  oz.  of  pure  mercury, 
and  agitate  the  jar  and  contents  for  from  i  to  2  hours.  Then 
carefully  separate  the  mercury  from  the  pulp  by  panning  in  a 
gold  pan  (it  may  be  necessary  to  add  more  Hg  to  the  pan  in 
order  to  collect  what  has  been  used),  saving  all  the  panned  pulp. 
This  is  dried,  thoroughly  mixed,  and  assayed  by  the  proper 
method.  The  mercury,  with  30  grams  of  lead,  is  placed  in  a 
2.5-in.  scorifier,  which  is  covered  with  a  second  one  turned  upside 
down,  into  the  bottom  of  which  a  small  hole  has  been  bored. 
The  scorifiers  are  luted  together  with  clay.  The  mercury  is  driven 
off  by  raising  the  temperature  gradually  to  a  bright  red.  The 
top  scorifier  is  then  removed,  the  lead  scorified  down  to  a  1 5-gram 
button,  which  is  cupeled,  and  the  gold  bead  weighed  and  parted. 
The  proper  calculation  is  then  made  for  the  value  per  assay  ton. 

The  Assay  of  Slags  and  Cupels  for  the  Correction  Assay.  — 
(a)  Slags:  The  charge  for  these  depends  upon  whether  they  are 
acid  or  basic.  Particular  care  must  be  taken  to  get  a  charge 
that  will  completely  decompose  the  original  slag.  If  this  is  acid, 
the  charge  should  aim  to  make  a  new  slag  more  basic,  and  vice 
versa.  The  lead  button  should  be  from  20  to  25  grams  in  weight. 
Many  assayers  frequently  add  simply  litharge  and  reducing  agent 


SPECIAL  METHODS  OF  ASSAY  1 19 

to  the  slag  in  making  the  fusion.  This  is  not  always  desirable, 
for  if  the  slag  already  has  much  litharge  in  it,  soda,  etc.,  may 
with  profit  be  added  as  the  extra  base  in  place  of  litharge. 

(b)  Cupels:  The  bone-ash  of  the  cupel  will  not  unite  with 
fluxes  to  form  slags,  but  remains  suspended  in  the  fusion.  For 
this  reason  the  cupel  should  be  put  through  a  150-  to  2oo-mesh 
screen  before  assaying,  the  litharge-stained  portion  only  being 
taken.  For  one  large  cupel,  or  two  small  ones,  the  charge  is  as 
follows: 

Cupel  Borax  glass   4$      grams 

PbO   100  grams         Argol    2.5  grams 

Na2CO3    25  grams         Salt  cover 

Fluorspar  is  not  desirable  in  the  assay  of  cupels,  as  it  merely 
adds  another  ingredient  in  suspension. 


XI 


ERRORS   IN  THE  ASSAY   FOR  GOLD  AND  SILVER 

Losses  in  tie  Cupellation  of  Pure  Gold  and  Silver.  —  These 
losses  may  be  divided  into  (i)  losses  by  absorption,  (2)  losses  by 
volatilization.  The  losses  of  gold  and  silver  in  the  cupellation 
are  functions  of  (a)  the  temperature  of  cupellation;  (V)  the  amount 
of  lead  with  which  the  gold  and  silver  is  cupeled;  (c)  the  physical 
nature  of  the  cupel;  (d)  the  nature  and  amount  of  impurities 
present;  (e)  the  influence  which  silver  has  on  the  gold  loss,  and 
vice  versa. 

There  is  considerable  literature  extant  upon  losses  in  cupella- 
tion of  the  two  precious  metals,  but  in  the  older  researches  the 
temperature  influence  is  but  vaguely  defined,  owing  to  the  lack 
of  means  for  ready  and  satisfactory  temperature  measurements, 
a  deficiency  which  is  now  supplied  by  the  Le  Chatelier  platinum- 
rhodium  pyrometer.  Losses  are  also  expressed  as  percentages  of 
the  total  amount  of  metal  cupeled,  and  then  the  average  per- 
centage losses  are  indicated.  That"  this  is  very  deceptive  is  made 
evident  by  reference  to  the  curve  of  losses  accompanying  this 
chapter. 

It  is  for  this  reason  that  the  statement  of  results  given  by 
Mason  and  Bowman,1  that  the  average  loss  in  cupellation  .of  pure 
silver  under  normal  conditions  is  1.99  per  cent,  and  for  gold 
0.296  per  cent.,  does  not  convey  any  very  definite  idea,  unless  the 
amount  of  metal  cupeled  is  accurately  specified,  as  well  as  the 
temperature.  This  fact  has  been  noted  by  other  observers,2  but 
no  effort  has  been  made  to  express  results  coordinately. 

The  following  data  show  the  losses  which  occur: 

1  "  Journ.  Am.  Chem.  Soc.,"  XVI,  p.  505. 

2  Kaufman,  in  "Eng.  and  Min.  Journ.,"  LXXIII,  p.  829.     Miller  and  Fulton, 
in  "Sch.  Mines  Quart.,"  XVII,  p.  169. 

120 


ERRORS  IN  ASSAY   FOR  GOLD  AND  SILVER 


121 


TABLE   XXI.  —  CUPELLATION   OF  PURE   SILVER 
(J.  EAGER  AND  W.  WELSH  ') 


AMT.  or  SILVER 

AMT.  OF  LEAD 

TEMPERATURE 

TOTAL  LOSSES 

MILLIGRAMS 

GRAMS 

DEC.  CENT. 

PER  CENT. 

204.62 

10 

700 

i  .  02  (average) 

205.00 

10 

775 

1.28 

203 

10 

850 

i-73 

203 

10 

925 

3-65 

203 

TO 

IOOO 

4.87 

TABLE   XXII.  —  CUPELLATION   OF  PURE   SILVER 

(L.  D.  GODSHALL2) 


AMT.  OF  SILVER 
MGS. 

AMT.  OF  LEAD 
GRAMS 

TEMP.  °  C. 
FRONT  OF 
MUFFLE 

TOTAL  Loss  IN  PER  CENT. 

2 

7-5 

75o°? 

3-66 

2 

15.0 

75° 

4.40 

2 

22.5 

750 

5-52 

2 

30.0 

75° 

5-96 

5 

7-5 

750 

3-29 

5 

15.0 

750 

2.63 

5 

22.5 

750 

3-83 

5 

30.0 

750 

4-31 

10 

7-5 

750 

3-73        - 

IO 

15.0 

750 

2.89  ' 

10 

22.5 

75° 

447 

10 

30.0 

750 

4.26 

20 

7-5 

75° 

342 

20 

15.0 

75° 

2-34 

20 

22.5 

750 

3-59 

20 

30.0 

750 

3.10 

50 

7-5 

75° 

2,14 

5° 

15.0 

75° 

2.46 

5° 

22.5 

75° 

2-33 

50 

30.0 

750 

2.89 

100 

7-5 

75o 

2.  II 

IOO 

15.0 

75° 

2.4O 

100 

22.5 

75o 

2.10 

IOO 

30.0 

75o 

2.28     • 

200 

7-5 

75° 

1.71 

200 

15.0 

75o 

1.64 

200 

22.5 

750 

1.62 

200 

30.0 

75o 

2.07 

Lodge,  "Notes  on  Assaying,"  p.  59.      2  Trans.  A.  I.  M.  E.,  Vol.  XXVI,  p.  473- 


122 


A   MANUAL  OF   FIRE  ASSAYING 


TABLE   XXIII.  —  CUPELLATION   OF  PURE   SILVER 

(KAUFMAN  !) 


AMT.  or  SILVER 
MGS. 

AMT.  OF  LEAD 
GRAMS 

TEMP.  °  C. 
FEATHERS 

TOTAL  Loss  IN  PER  CENT. 

25 

5 

75o°? 

2.14 

25 

10 

75° 

2.63  (2.38,  2.43) 

25 

15 

75° 

2.69 

25 

25 

75° 

2.09  (2.48,  2.44) 

5° 

5 

75° 

1-43 

5° 

10 

75o 

2.23  (2.10,  1.96) 

5° 

15 

75° 

2.14 

50 

25 

750 

1.86  (2.25,  2.37) 

100 

5 

75° 

1.30 

IOO 

10 

75° 

1.61  (1.82,  1.42) 

100 

15 

75° 

1.68 

IOO 

25 

75° 

2.12  (1.93,  2.12) 

200 

5 

75° 

0.86 

200 

10 

75o 

1.24  (1.29,  1.17) 

200 

i5 

75o 

1.40 

200 

25 

75° 

1.74  (1.46,  1.76) 

Parentheses  indicate  different  types  of  cupels,  e.g.,  bone-ash 
made  up  respectively  with  pearl-ash  and  stale  beer.  The  main 
figures  were  obtained  by  bone-ash  cupels  made  up  with  water. 
The  results,  viewed  as  a  whole,  indicate  that  all  three  types  have 
equal  merit.  Godshall  (Table  XXII)  experimented  with  different 
types  of  standard  bone-ash  cupels  (some  made  at  the  mint),  with 
the  same  result.  Rose,  cited  further  on,  shows  that  magnesia 
cupels  cause  somewhat  greater  losses  than  good  bone-ash  cupels. 

The  agreement  amongst  the  different  writers  is  very  close, 
when  the  fact  is  taken  into  consideration  that  in  the  last  two 
cases  no  precise  statement  concerning  temperature  is  made,  and 
that  the  amounts  of  lead  differ  somewhat. 


AMT.  OF 


TOTAL  Loss 


AUTHORITY 


SILVER        AMT.  OF  LEAD    TEMPERATURE         PER  CENT. 


Mgs. 

Grams 

Eager  and  Welch 

205 

10 

Godshall 

200 

7 

Kaufman  . 

200 

/  • 

10 

Degrees  Cent. 

775 

?  (feathers) 
?  (feathers) 


1.28 
1.24 


"Eng.  and  Min  Journ.,"  LXXIII,  p.  829. 


ERRORS   IN   ASSAY    FOR  GOLD   AND   SILVER 


123 


The  accompanying  curves  are  constructed  from  figures  in 
Mr.  Godshall's  paper.  The  general  averages  are  taken,  and 
while  his  losses  are  perhaps  a  trifle  higher  than  the  best  work 
calls  for  at  the  present  day- (owing  to  a  better  recognition  of  the 
precise  temperature  required),  they  form  the  best  and  most 
complete  data  for  the  construction  of  curves  showing  the  relation 
between  amounts  of  silver  cupeled  and  the  percentage  loss. 
I  refrain  from  a  mathematical  discussion,  but  an  equation  covering 
the  case  is  tentatively  offered.1 

The  influence  of  the  size  of  lead  button  is  clearly  discernible 
by  the  ordinates  of  the  curves.  The  temperature  variations  will 
show  in  the  same  way. 

TABLE   XXIV.  —  CUPELLATION   OF   GOLD 
(EAGER  AND  WELCH  2) 


AMT..OF  GOLD 

AMT.  OF  LEAD 

TEMP.  C. 

TOTAL  Loss  PER  CENT. 

201 

10 

775° 

o-i55 

201 

10 

850 

0.430 

204 

10 

925 

0.460 

201 

10 

IOOO 

i  .  430 

201 

10 

1075 

3.000 

1 1  am  indebted  to  Mr.  C.  C.  Van  Nuys,  B.  S.,  for  the  curves  and  the  equations. 
2  Lodge,  "Notes  on  Assaying,"  p.  142. 


Per-cent  of  Loss 


-Id 

-W 


>* 


-S 


-S 


ERRORS    IN  ASSAY    FOR  GOLD  AND   SILVER 


125 


TABLE   XXV.  —  CUPELLATION   OF   GOLD 

(HlLLEBRAND  AND  ALLEN  !) 


AMT.  OF 

AMT.  OF 

TEMP.  °  C. 

TOTAL  Loss 

TOTAL  Loss 

TOTAL  Loss 

GOLD 

LEAD 

PER  CENT. 

CUPEL 

VOLATILIZED 

ABSORPTION 

MGS. 

GRAMS. 

PER  CENT. 

PER  CENT. 

30-58 

25 

750°?  feathers 

0.36 

30-32 

25 

increased 

1.19 

80 

20 

30-63 

25 

increased 

1.76 

3°  -45 

25 

increased 

3-78 

77 

23 

30.16 

25 

increased 

4.17 

93 

7 

30.66 

25 

back  of  muffle 

4-43 

92 

.  8 

10.34 

25 

750°?   front  of 

0.29 

muffle 

10.25 

25 

increased 

4.68 

10.29 

25 

increased 

1.36 

10.27 

25 

increased 

10.42 

10.  17 

25 

back  of  muffle 

16.43 

The  literature  of  gold  losses  is  considerably  less  than  that  for 
silver.  Rose 2  discusses  them  in  the  gold  bullion  assay.  He 
gives  the  total  loss  on  bullion  916.6  fine,  under  normal  tempera- 
ture conditions,  as  from  0.4  to  0.8  per  1000,  of  which  82  per  cent, 
is  cupel  absorption,  10  per  cent,  volatilization  (probably),  and 
8  per  cent,  solution  in  acid.  This,  calculated  to  percentage  on 
actual  gold,  is  equivalent  to  0.0803  per  cent,  for  the  highest 
loss.  (This  is  cupel  loss  only,  not  including  solution  loss.) 

Hillebrand  and  Allen's  results  contain  interesting  data  re- 
garding the  relative  losses  by  absorption  and  volatilization,  to 
which  reference  will  be  made  again. 

Cupellation  of  Gold-Silver  Alloys.  —  The  loss  of  gold  and  silver 
in  cupellation  is  somewhat  different  when  both  gold  and  silver 
are  present  from  the  loss  when  either  metal  alone  is  present. 

1  Bull.  No.  253,  U.  S.  G.  Survey. 

2  "Metallurgy  of  Gold,"  1902,  p.  506. 


126 


A   MANUAL  OF   FIRE  ASSAYING 


TABLE   XXVI.  —  CUPELLATION   OF  GOLD 
(ROSE  i) 


AMT.  OF  GOLD 
MGS. 

AMT.  OF  SILVER 
MGS. 

AMT.  OF  PB 
GRAMS 

TEMP.  OF  CUPEL- 
LATION °  C. 

TOTAL  Loss  GOLD 
PER  CENT 

i 

4 

25 

900° 

I  .  2 

i 

6 

25 

900 

1.05 

i 

8 

10 

900 

0.90 

i 

10 

25 

900 

0.80 

i 

6 

25 

700 

0-45 

i 

10 

25 

700 

o-39 

500 

1250 

10 

900 

°-055 

TABLE   XXVII.  —  CUPELLATION   OF   GOLD-SILVER  ALLOYS 

(HlLLEBRAND  AND  ALLEN) 
ALL  CUPELLATIONS  MADE  WITH  25   GRAMS  OF  LEAD 


TOTAL  LOSS 

ABSORBED  BY 
CUPEL 

VOLATILIZED 

AMOUNT 

AMOUNT 

GOLD 

SILVER 

TEMP.  °  C. 

MGS. 

MGS. 

Gold 

Silver 

Gold 

Silver 

Gold 

Silver 

Per 

Per 

Per 

Per        Per 

Per 

cent. 

cent. 

cent. 

cent.       cent. 

cent. 

30.06 

90.51 

750? 

0.50 

1.70 

30.40 

90.19 

increased 

I  .  22 

3-73 

30.60 

90.74 

increased 

2.32 

5-5i 

30.07 

90.67 

increased 

3-76 

7.66 

30.61 

90.75 

back  of  muffle 

3.89 

7.98 

iS-56 

45.06 

front,  750? 

o.  19 

I.  01 

67 

81           33 

19 

i5-i4 

45-J9 

increased 

0.40 

3-3° 

83 

78           17 

22 

i5-i5 

45-41 

increased 

I-52 

4.14 

52 

63 

48 

37 

15-44 

45-3° 

increased 

2.07 

5-78 

67 

62           33 

38 

iS-S2 

45-59 

increased 

2-59 

6-55 

67 

7o          33 

3° 

15-39 

45-05 

back  of  muffle 

2  .40 

6.61 

73 

71           27 

29 

10.67 

30.33 

front,  750? 

0.47 

2.17 

10.57 

30.64 

increased 

1.61 

5.68 

io-53 

30.42 

increased 

5-i3 

10.19 

10.63 

30-52 

increased 

10.63 

15-99 

10.  60 

30-38 

increased 

12.46 

18.34 

10.21 

30-44 

back  of  muffle 

12.53 

18.69 

"Eng.  and  Min.  Journ.,"  LXXIX,  p.  708. 


ERRORS    IN   ASSAY    FOR  GOLD  AND   SILVER          127 

Rose  shows  (Table  XXVI)  the  protective  action  that  silver 
exercises  over  gold,  the  total  loss  of  gold  decreasing  as  the  amount 
of  silver  present  increases.  Hillebrand  and  Allen  show  how  the 
total  loss  is  distributed  between  absorption  by  the  cupel  and 
volatilization.  It  is  evident  that  while  the  total  loss  of  gold  is 
increased  by  the  presence  of  silver,  the  volatilization  loss  of  gold 
is  increased  by  the  presence  of  silver  (compare  Tables  XXV  and 
XXVII).  When  gold  and  silver  are  present  in  the  ratio  of  i  to 
2,  the  averages  are  as  follows: 

Of  the  total  gold  loss,  68  per  cent,  is  absorbed,  32  per  cent,  is 
volatilized.  \ 

Of  the  total  silver  loss,  71  per  cent,  is  absorbed,  29  per  cent. 
is  volatilized. 

However,  as  the  total  loss  is  determined  by  the  difference  in 
weight  between  the  proof  gold  and  silver  and  the  weights  of  the 
cupeled  bead  and  parted  gold,  and  the  volatilization  item  by 
the  difference  between  the  total  loss  and  the  amount  recovered 
by  the  re-assay  of  the  cupel,  it  is  evident  that  certain  errors  obtain 
which  apparently  make  the  volatilization  loss  appear  greater 
than  it  really  is.  The  error,  however,  cannot  be  very  great. 
The  data  are  inconclusive  regarding  the  influence  of  the  tempera- 
ture on  the  relative  losses  by  absorption  and  volatilization,  but 
it  seems  indicated  that  the  volatilization  loss  is  proportionately 
greater  with  an  increase  of  the  temperature  of  cupellation. 

Table  XXVIII  etc.,  show  losses  of  gold  and  silver  in  the  assay 
of  ores,  during  fusion  and  cupellation,  as  influenced  by  the  pres- 
ence of  certain  impurities. 


128 


A  MANUAL  OF   FIRE  ASSAYING 


TABLE   XXVIII.  —  TELLURIDE    ORES 


AMOUNT  or  GOLD  IN 
WEIGHT  OF  ORE 
TAKEN  FOR  ASSAY 

WEIGHT 
OF  LEAD 
BUTTON 

METHOD  OF 
FUSION 

SLAG  Loss 

CUPEL 
ABSORPTION 

Milligrams 

Grams. 

Per  cent. 

Per  cent. 

O493-83 

20 

Crucible7 

0.51 

1.56 

95-57 

20 

Crucible 

0.38 

0-23  (5) 

i-54 

20 

Crucible 

1.30 

0.40 

i-95 

20 

Crucible 

o  .  50 

o  .  ^o 

1.19 

20 

Crucible 

0.40 

1.17 

20 

Crucible 

0.40 

3.60 

20 

Crucible 

2.14 

0.80 

6.20 

20 

Crucible 

0.64 

0.32 

6.23 

20 

Crucible 

0.50 

0.64 

1.38 

2O 

Crucible 

0.80 

I  .00 

0)  18.18 

25 

Crucible 

0.49 

5-85 

25 

Crucible 

1.03 

O.I2(6) 

(3)     34-o 

27 

Crucible 

0.21 

0.23 

34-o 

27 

Crucible 

0.56 

34-0 

25 

Crucible 

0.15 

0.41 

68.0 

25 

Crucible 

0.13 

0.07 

68.0 

25 

Crucible 

o.  16 

0.  22 

(4)     15-5 

Crucible8 

0.25 

o.  19 

15-49 

Crucible 

0.13 

0.38 

J9-54 

Crucible 

0.20 

0.23 

19.63 

Crucible 

0.  10 

0.25 

1  Woodward,  in  "West.  Chem.  and  Met."  I,  p.  12. 

2  Fulton,  in  "Sch.  Mines  Quart.,"  XIX,  p.  419. 

3  Lodge,  in  "Tech.  Quart.,"  1899,  XII,  p.  171  (averages). 

4  Bull.  No.  253,  U.  S.  G.  Survey  (averages;  Hillebrand  and  Allen). 

5  Average  of  34  fusions,  tellurium  in  all  beads. 

6  Average  of  10  fusions. 

7  Cripple  Creek  flux. 

8  Excess-litharge  charge. 


ERRORS   IN  ASSAY    FOR  GOLD   AND   SILVER          129 
TABLE  XXIX.  —  ZINCIFEROUS   MATERIAL,   ETC. 


AMT.  OF  Au  AND 
Ac  IN  WEIGHT  OF 

ORE  TAKEN  FOR 

WEIGHT 
OF  LEAD 
BUTTON 

METHOD 

SLAG  Loss 

CUPEL 
ABSORPTION 

ASSAY 

OF 

REMARKS 

Au 
Mgs. 

Ag 

Mgs. 

Grams 

Au        Ag 
Per       Per 
cent.  |  cent. 

Au 
Per 
cent. 

Ag 
Per 
cent. 

1  232.0 

287.0 

18 

Scorification     after 

0.06 

0.40 

O.II 

1-30 

Zn.     ppt.    containing 

acid  treacment 

42.3  per  cent.  Zn 

232.0 

284.0 

18 

Scorification     after 

0.04      0.34 

0.08 

I.IO 

Figures  represent 

acid  treatment 

averages 

232.0 

284.0 

18 

Direct  crucible 

1.04      no 

0.16 

1.50 

fusion 

2  233.0 

197.0 

20 

Crucible   fusion 

0.06      0.51 

0.18 

1.18 

Zn     ppt.     containing 

after   acid    treat- 

14.3  per    cent.   Zn, 

ment 

9.1   per  cent.  Cu. 

233-0 

2O  2.  0 

20 

Direct  crucible 

0.15 

2-73 

0.16 

1.29 

Figures  represent 

fusion 

averages 

3 

56l.O 

20 

Crucible  fusion 

o-75 

1.38 

Galena 

niter  method 

567.0 

2O 

Crucible  fusion 

0.65 

1.42 

Galena 

niter  method 

3 

175-0 

20 

Crucible  fusion 

0.23 

1.90 

Silicious  ore  contain- 

niter method 

ing  some  copper 

174.0 

20 

0.37 

1.66 

Silicious  ore  contain- 

ing some  copper 

1  Fulton  and  Crawford,  in  "Sch.  Mines  Quart.,"  XXII,  p.  153. 

2  Lodge,  in  Trans.  A.  I.  M.  E.,  XXXIV,  p.  432. 

3  Miller,  in  "Sch.  Mines  Quart.,"  XIX,  p.  43. 


A  MANUAL  OF   FIRE  ASSAYING 


TABLE  XXX.i  —  HIGH-GRADE  CARBONATE  AND   SULPHIDE 
SILVER   ORES  2 


AMT.  OF  Au  AND 
Ac  IN  WEIGHT  OF 

ORE  TAKEN  FOR 

WEIGHT 
OF  LEAD 
BUTTON 

METHOD 

SLAG  Loss 

CUPEL 

ABSORPTION 

SECOND   CORRECTION 
FROM  FUSION  OF 

ASSAY 

OF 

SLAGS  AND  CUPELS 

. 

OF  FIRST  CORRECTION 

Au 
Mgs. 

Ag 
Mgs. 

Grams 

Au 
Per 
cent. 

Ag 
Per 
cent. 

Au 
Per 
cent. 

Ag 
Per 
cent. 

PER  CENT. 

1 

1130 

24 

Crucible  fusion 

0.76 

1-25 

0.248 

1130 

28 

Crucible  fusion 

0.361 

0.956 

(  Crucible  fusion 

1130 

28 

<  Double   amt.    of 

0.511 

0.88 

0.15 

(       fluxes 

(  Crucible  fusion 

1130 

32 

\  Double   amt.    of 

0.736 

1.02 

0.15 

(       fluxes 

226 

is 

Scorification 

2.8 

I.40 

0-559 

24 

1719 

25 

Crucible  fusion 

0.05 

0.143 

0.07 

0.7II 

12 

860 

20 

Scorification 

0.12 

0.72 

0.08 

0.86 

TABLE  XXXI.  —  CUPRIFEROUS   MATERIAL 


AMT.  OF  Au  AND 
Ac  IN  WEIGHT  OF 

ORE  TAKEN  FOR 

WEIGHT 
OF  LEAD 
BUTTON 

METHOD 

TOTAL  Loss   RECOVERED 
INCLUDING 
SLAG  AND  CUPEL 

ASSAY 

OF 

REMARKS 

. 

Au 
Mgs. 

Ag 
Mgs. 

Grams 

Au 
Per  cent. 

Ag 
Per  cent. 

11.46 

88.0 

20 

Crucible  fusion 

0.96 

2.90 

Mattes  containing 

about  20  per  cent.  Cu 

5-36 

46.8 

20 

Crucible  fusion 

2.98 

4-3° 

4.20 

37-24 

20 

Crucible  fusion 

2-38 

2.70 

3-52 

33-79 

20 

Crucible  fusion 

2.84 

5-47 

4.20 

37-24 

20 

Scorification 

5-95 

5-26 

3-52 

33-79 

20 

Scorification 

3.12 

5-71 

The  foregoing  tables  represent  for  the  most  part  averages, 
and  in  every  case  the  losses  for  the  normal  assay;  e.g.,  in  the  case 
of  the  fusion,  the  charge  known  to  yield  the  best  results,  and 
the  proper  temperature  for  cupellation.  The  losses  are  therefore 

1  First  five  results  on  lead  carbonate  ore,  last  two  on  silver  sulphides.     All 
results  represent  averages. 

2  Miller  and  Fulton,  ibid.,  XVII,  p.  160. 


ERRORS    IN   ASSAY    FOR  GOLD  AND   SILVER          131 

to  be  ascribed  to  the  nature  of  the  material  assayed,  chiefly  to 
the  influence  of  certain  elements  present.  In  considering  the 
percentage  of  loss,  it  must  be  recalled  that  this  varies  inversely 
with  the  amount  of  precious  metal  in  the  charge,  i.e.,  with  the 
size  of  the  gold-silver  bead.  The  sum  of  the  cupel  absorption 
and  the  slag  loss  (which  can,  in  part,  be  recovered)  is  not  the 
total  loss,  as  it  does  not  include  that  by  volatilization,  which  is 
small  in  most  cases,  but  in  some  cases,  again,  may  be  quite  appre- 
ciable, as  in  the  case  of  telluride  ores.  What  the  loss  is  in  slag, 
when  no  element  like  tellurium,  copper,  zinc,  etc.,  is  present,  may 
be  seen  by  reference  to  Table  XXIX,  to  those  assays  fused  after 
acid  treatment,  and  to  Table  XXX,  showing  crucible  fusions  on 
lead  carbonate  ore.  The  slag  loss  in  gold  and  silver  for  these  ores 
is  very  small.  In  cases  where  the  impurity  present  and  causing 
loss  is  nearly  all  eliminated  in  the  fusion,  e.g.,  zinc,  antimony, 
etc.,  the  cupel  absorption  is  practically  that  for  pure  silver  and 
gold  under  the  same  circumstances.  Where  the  impurity  is 
tellurium,  or  selenium,  or  copper,  the  cupel  absorption  is  decidedly 
increased.  One  fact  is  to  be  noted,  the  fact  that  the  slag  losses 
present  no  regularity,  even  for  the  same  material.  This  is  prob- 
ably due  partly  to  differences  of  slag  composition  among  different 
experimenters,  and  partly  to  difference  of  temperature  of  fusion; 
and  also  to  the  method  of  refusion  of  slag. 

The  high  loss  in  scorification  slags  shown  in  Table  XXX  for  lead 
carbonate  ores  containing  silver  is  due  to  the  general  unsuitability 
of  the  ore  for  scorification,  although  scorification  slags  show  higher 
losses  than  crucible  slags.  That,  in  spite  of  this,  scorification 
assays  on  silver-bearing  material  show  equally  good  and  better 
results  in  many  cases  than  the  crucible  assay,  is  due  to  the  fact 
that  the  silver  beads  retain  small  quantities  of  lead  and  copper 
(see  further  on),  and  to  the  fact  that  in  the  multiplication  of  the 
weight  of  the  silver  bead  by  5  or  10,  or  whatever  the  assay-ton 
factor  may  be,  this  error  is  multiplied,  giving  an  apparently 
better  result. 

The  amount  of  slag  has  comparatively  little  influence  on  the 
amount  of  precious  metals  retained,  provided  the  amount  of 
collecting  lead  is  ample.  Buttons  of  less  than  "1 8  to  20  grams 
should  not  be  made,  and  if  the  amount  of  slag  is  great  or  the 
quantity  of  silver  and  gold  in  the  charge  is  more  than  500  mgs., 


132  A  MANUAL  OF   FIRE   ASSAYING 

25-  and  3O-gram  buttons  are  essential.  In  the  case  of  large 
buttons  which  contain  no  impurity,  it  is  also  best  to  cupel  direct, 
if  possible,  rather  than  rescorify  to  smaller  size,  as  the  rescorifi- 
cation  causes  greater  loss  than  the  direct  cupellations. 

During  scorification  there  is  also  an  appreciable  loss  of  the 
precious  metals  by  volatilization,  which  is  absent  in  the  crucible 
assay.  This,  in  the  case  of  telluride  or  zinciferous  ores,  may 
become  so  great  as  to  put  scorification  out  of  the  question. 

Other  Errors:  Retention  of  Lead  in  Cupeled  Beads.  —  Small 
quantities  of  lead  are  almost  invariably  retained  in  the  gold  and 
silver  beads  with  ordinary  temperatures  of  cupellation.  Hille- 
brand  and  Allen,1  in  two  careful  experiments  on  sets  of  three 
beads,  approximately  together  90  mgs.  gold,  found  that  0.30  per 
cent,  and  0.37  per  cent.,  respectively,  of  lead  were  retained. 
This  retention  of  lead  cannot  be  corrected  by  leaving  the  bead 
in  the  muffle  for  some  length  of  time  after  the  blick,  as  this  is,  of 
course,  prohibitive  in  the  case  of  silver,  and  in  the  case  of  gold 
seems  to  actually  cause  an  increase  of  weight.  It  has  already 
been  stated  that  copper  and  tellurium  are  very  apt  to  be  present 
in  the  final  bead,  when  in  the  ore  in  any  appreciable  quantity. 
The  retention  of  base  metal  by  the  bead  causes  a  plus  error  in 
silver,  but  will  not  effect  the  result  on  gold  unless  the  parting  is 
by  H2SO4;  and  where  the  weight  of  the  bead  is  multiplied  by  a 
factor  to  get  results  per  ton,  the  final  error  in  silver  may  be  very 
appreciable.  The  presence  of  copper  in  the  final  bead  practically 
insures  the  complete  removal  of  the  lead. 

Retention  of  Silver  by  the  Parted  Gold.  —  Ordinary  parted  gold, 
after  the  proper  treatment  with  weak  and  strong  acid,  retains 
from  0.05  to  o.  10  per  cent,  of  silver.  In  the  assay  of  gold  bullion, 
after  the  first  acid  treatment  of  the  quartation  alloy,  the  gold 
on  the  average  retains  0.25  per  cent,  silver.  After  the  second 
acid  treatment,  the  final  silver  retention  is  from  0.06  to  0.09 
per  cent.,  depending  on  the  time  of  boiling.  If  the  amount  of 
silver  to  gold  in  the  quartation  alloy  is  less  than  2.5  to  i,  some- 
what more  than  the  above  amount  of  silver  will  be  retained.2 

Silver  can,  practically,  be  completely  extracted  by  more  than 
two  treatments  with  acids,  according  to  Hillebrand  and  Allen.3 

1  Bull.  No.  253,  U.  S.  G.  Survey.  2  Rose,  "Metallurgy  of  Gold,"  p.  453. 

3  Bull.  253,  U.  S.  G.  Survey. 


ERROR   IN   ASSAY    FOR  GOLD  AND   SILVER  133 

In  the  ordinary  assay  for  ores  as  usually  carried  out,  it  is  safe  to 
assume  that  some  silver  is  invariably  retained  by  the  gold,  and 
frequently  much  more  than  is  supposed;  however,  with  low-grade 
ores,  this  retention  is  negligible. 

Solution  of  Gold  by  Acid.  —  It  is  essential  that  the  nitric  acid 
used  for  parting  be  free  from  impurities,  especially  from  hydro- 
chloric acid  and  chlorine;  otherwise  solution  of  gold  is  sure  to 
follow.  Gold  is  quite  soluble  in  mixtures  of  hot  sulphuric  and 
nitric  acid,1  and  is  again  precipitated  by  dilution. 

According  to  Hillebrand  and  Allen,2  nitrous  acid  (HNO2)  and 
mixtures  of  BNO3  and  HNO2  do  not  dissolve  gold,  though  there 
is  much  earlier  literature  to  the  contrary.  Nitrous  acid  has 
frequently  been  considered  in  this  connection,  as  it  is  formed  to 
some  extent  by  the  action  of  HNO3  on  silver. 

According  to  Rose,3  some  gold  is  dissolved  by  nitric  acid  on 
continued  boiling  to  constant  gravity  of  acid.  This  solution  is 
placed  in  the  bullion  assay  at  0.05  per  cent,  or  0.5  parts  per  1000. 
Hillebrand  and  Alien  state  that  the  loss  of  gold  by  solution  is 
very  small  and  irregular.  It  may  be  disregarded  in  the  ore  assay. 

Occluded  Gases.  —  Parted  gold  beads  and  "cornets"  retain 
about  twice  their  volume  in  occluded  gases  after  annealing.  The 
principal  gas  is  stated  to  be  carbon  monoxide.  Two  volumes 
amount  to  0.02  per  cent,  by  weight,  which  is  already  allowed  for 
in  the  silver  retention. 

Errors  in  Weighing.  —  The  best  scales  are  accurate  to  o.oi  mg., 
and  scales  can  be  obtained  weighing  to  0.005  mg.  This  last  is 
used  in  assay  offices,  where  very  great  accuracy  is  required,  on 
such  material  as  bullions,  rich  mattes,  etc.  It  is  usually  an 
unnecessary  refinement  in  the  ordinary  ore  assay,  for  the  reason 
that  the  probable  error  in  the  assay  is  greater  than  this. 

The  errors  in  the  assay  for  gold  and  silver  may  be  summarized 
as  follows: 

1.  Losses  by  absorption  in  the  slag  of  the  fusion. 

2.  Losses  by  volatilization  during  fusion. 

3.  Losses  by  absorption  during  cupellation. 

4.  Losses  by  volatilization  during  cupellation. 

1  Lenher,  in  "  Journ.  Am.  Chem.  Soc.,"  XXVI,  p.  552. 

2  Ibid. 

3  Ibid.,  p.  507. 


134  A  MANUAL  OF   FIRE  ASSAYING 

5.  Errors  by  gain  in  weight  of  bead,  due  to  retention  of  foreign 
elements.     This  affects  results  on  silver  chiefly. 

6.  Errors  in  weight  of  gold  after  parting  by  the  retention  of 
silver  and  occluded  gases. 

7.  Losses  of  gold  by  solution  in  nitric  acid. 

8.  Errors  in  weighing. 

The  chief  losses  are  Nos.  i  and  3,  which  can  be  recovered  by 
"corrected  assay,"  i.e.,  by  re-assay  of  slag  and  cupel,  to  the  extent 
of  about  80  to  85  per  cent.  Wherever  considerable  accuracy  is 
required,  corrected  assays  should  always  be  made.  The  losses 
by  volatilization  are  usually  slight,  although  from  the  foregoing 
data  these  are  sometimes  seen  to  be  considerable.  The  retention 
of  foreign  metals  by  the  bead  is  a  plus  error  in  favor  of  silver, 
and  the  retention  of  silver  in  the  parted  gold  is  a  plus  error  in 
favor  of  gold.  Silver  losses  are  considerably  greater  in  magnitude 
than  gold  losses.  The  total  amount  of  precious  metal  recovered 
by  the  assay  varies  with  the  nature  of  the  material.  Designating 
the  total  amount  of  gold  and  silver  in  an  ore  or  product  as  100, 
the  corrected  assay  will  show  from  99  to  99.8  per  cent,  of  the 
gold,  and  from  98  to  100  +  per  cent,  of  the  silver,  the  high  silver 
result  in  some  cases  being  due  to  retention  of  foreign  metal. 

In  the  bullion  assay  for  gold,  the  algebraic  sum  of  the  errors 
outlined,  the  losses  being  designated  minus  and  the  gains  plus, 
is  called  the  ''surcharge/'  In  the  gold  bullion  assay  this  will 
vary  from  +0.025  per  cent,  in  very  pure  gold  bullion,  to  0.25 
per  cent,  in  base  bullion,  passing  to  zero  for  a  bullion  about 
800  fine. 


XII 


THE  ASSAY  OF  BULLION 

General.  —  Bullion  is  classified  as  follows: 

1.  Lead  bullion,  usually  the  product  of  the  lead  blast-furnace; 
95  per  cent,  and  more  lead,  containing  some  copper,  antimony, 
etc.,  silver  and  gold. 

2.  Base  bullion,  containing  from  100  to  925  parts  of  silver 
per  1000,  gold  in  varying  amounts,  and  a  large  percentage  of 
base   metals,    chiefly   copper,    zinc,    lead,   etc.     Produced   most 
frequently  by  cyanide  mills. 

3.  Dore  bullion,  containing  925  to  990  parts  of  silver  per 
1000,  some  gold,  and  base  metals,  mostly  copper,  but  also  lead, 
antimony,  zinc,  etc. 

4.  Fine  silver  bullion,  free  from  gold,  containing  990  to  1000 
parts  silver  per  1000,  but  some  base  metals,  usually  copper. 

5.  Silver  bullion,  containing  little  base  metal  and  less  than 
half  its  weight  in  gold. 

6.  Gold  bullion,  containing  little  base  metal  and  more  than 
half  its  weight  in  gold. 

7.  Fine  gold  bullion,  free  from  silver,  containing  from  990  to 
1000  parts  gold  per  1000. 

Silver  and  gold  in  all  bullions  but  lead  bullion  are  estimated 
in  parts  per  thousand,  and  bullion  is  said  to  be  so  many  parts 
fine.  Thus,  if  i  gram  (1000  mgs.)  of  bullion  is  taken  for  assay 
and  it  contains  925  mgs.  gold,  it  is  said  to  be  925  fine. 

In  the  assay  of  gold  bullion  the  "millieme"  system  of  assay 
weights  is  used,  a  millieme  being  0.5  mg.,  and  the  assay  is  reported 
in  parts  of  10,000,  or  the  fineness  with  one  decimal  added.  Thus 
the  above  bullion  would  be  reported  as  925.0  fine.  In  this  system 
the  5OO-mg.  weight  is  stamped  1000,  the  25O-mg.  weight  500,  etc. 
The  scales  used  must  therefore  be  sensitive  to  0.05  mg.,  or  o.i 
millieme.  This  presents  no  difficulty,  as  ordinary  assay  balances 
are  sensitive  to  o.oi  mg.  with  a  load  of  0.5  gram. 


136  A   MANUAL  OF   FIRE  ASSAYING 

Lead  bullion  is  recorded  in  oz.  per  ton,  in  the  same  way  as 
for  ores. 

The  Assay  of  Lead  Bullion. — The  sample  of  bullion  may  be 
melted  under  charcoal  and  granulated  in  cold  water,  or  it  may 
be  rolled  out  into  a  strip  in  the  rolls,  and  the  pieces  cut  at  intervals 
from  this  for  the  sample.  If  lead  bullion  is  free  from  copper, 
antimony,  zinc,  sulphur  and  arsenic,  etc.,  it  may  be  cupeled 
directly  for  gold  and  silver.  In  this  case,  4  portions  of  0.5  assay 
ton  each  are  wrapped  in  about  7  grams  of  sheet  lead,  placed  in 
the  hot  cupels,  and  cupeled  with  feathers.  The  cupels  are  fused 
with  the  following  charge: 

Stained  part  of  cupel  45  grams  borax  glass 

80  grams  PbO  2  grams  argol 

15  grams  NaaCOa  Thin  litharge  cover 

The  buttons  from  this  fusion  are  cupeled  and  the  weight  of 
the  gold  and  silver  added  to  that  obtained  from  the  first  cupella- 
tion. 

If  the  bullion  contains  base  metals  which  will  influence  the 
results  of  the  cupellation,  4  portions  of  either  0.5  or  i.o  assay 
ton  are  weighed  out  and  mixed  with  30  to  50  grams  of  test  lead; 
1.5  grams  of  borax  glass  and  0.5  gram  of  silica  are  put  on  top  of 
the  lead  and  the  charge  scorified.  The  resultant  buttons,  which 
should  weigh  about  15  grams,  are  then  cupeled.  The  scorifier 
slag  and  cupel  are  re-assayed  by  the  above  charge  and  the  correc- 
tion added. 

The  Assay  of  Silver  Bullion  1  (also  applicable  to  Base  Bullion, 
Dore  Bullion,  etc.).  Cupellation  Method. — This  method  is  used 
as  an  approximation  for  bullions  in  which  silver  is  to  be  determined 
accurately,  serving  as  a  preliminary  assay  for  the  salt  titration, 
mint,  or  Gay  Lussac  method. 

(a)  Preliminary  Assay.  —  Exactly  500  mgs.  of  bullion  are 
weighed  out  on  an  assay  balance  in  order  to  save  calculation, 
wrapped  in  10  grams  of  sheet  lead,  and  cupeled  at  700°  C,  or 
with  ample  feathers  of  litharge.  The  silver  bead  is  cleaned, 
weighed  and  parted  in  i  to  9  HNO3  for  at  least  20  minutes;  then, 
if  any  gold  shows,  heated  for  5  minutes  more  in  concentrated 
acid,  washed,  and  the  gold  dried,  annealed  and  weighed.  The 

1  For  sampling  of  silver  bullion,  see  "The  Assay  of  Gold  Bullion,"  later  in 
this  chapter. 


THE  ASSAY   OF   BULLION 


137 


amount  of  gold  found,  subtracted  from  the  weight  of  the  bead, 
gives  the  approximate  silver,  and  the  weight  of  the  bead,  sub- 
tracted from  the  amount  of  bullion  taken  (500  mgs.),  gives  the 
base  metal.  This  base  metal  is  usually  copper,  and  its  presence 
may  be  detected  by  the  coloring  of  the  cupel. 

(b)  Making  the  Check  Assay.  —  As  the  loss  of  silver  and  gold 
is  a  question  of  temperature,  amount  of  precious  metal  present, 
amount  of  lead  of  cupellation,  and  amount  and  kind  of  base 
metal  present,  it  is  desirable  to  have  the  regular  cupellation, 
accompanied  by  a  check  assay,  made  up  as  nearly  as  possible  to 
the  composition  of  the  bullion  to  be  assayed,  and  cupeled  under 
the  same  conditions.  The  check  assay  is  therefore  made  up  from 
data  obtained  in  the  preliminary  assay.  As  the  silver  determined 
in  this  preliminary  assay  is  low,  due  to  absorption  and  volatiliza- 
tion, a  correction  of  1.2  per  cent,  is  added  as  an  approximation 
or,  rather,  the  amount  of  Ag  found  is  considered  as  98.8  per 
cent,  of  that  present),  and  this  amount  of  proof  silver  weighed 
out.  To  this  is  added,  in  proof  gold,  the  amount  of  gold  found 
in  the  preliminary  assay.  The  difference  between  the  sum  of 
the  corrected  silver  and  the  gold,  and  500,  is  the  amount  of  base 
metal  to  be  weighed  out  for  the  check.  As  already  stated,  the 
base  metal  is  usually  copper,  and  in  making  up  the  check  c.p. 
sheet  copper  is  used.  The  check  thus  weighs  500  mgs.  and  approx- 
imates very  closely  the  composition  of  the  bullion.  Duplicates 
of  500  mgs.  of  bullion  are  now  weighed  out,  and  these  and  the 
check  each  wrapped  in  the  proper  amount  of  sheet  lead,  as  de- 
termined from  the  table  below: 

TABLE   XXXII.  — LEAD    RATIO   IN   CUPELLATION 


FINENESS  IN 

AMOUNT  OF 

AMOUNT  OF  LEAD 

SILVER 

COPPER  PRESENT 

FOR  CUPELLATION 

RATIO  OF  LEAD  TO 

BASE  ]Vf  ETAL 

Milliemes 

Milliemes 

Grams 

IOOO 

0 

3 

900 

TOO 

7 

70  to  i 

800 

200 

12 

60  to  i 

S°° 

500 

18 

36  to  i 

300 

700 

21 

30  to  i 

138  A  MANUAL  OF   FIRE  ASSAYING 

(c)  The  Assay.  — Three  cupels  are  placed  in  a  row  across  the 
muffle,  so  as  to  be  exposed  as  nearly  as  possible  to  the  same 
temperature,  and  three  more  cupels  are  placed  near  them  to  act 
as  covers  for  the  cupellation  when  finished,  in  order  to  prevent 
sprouting.  When  the  .cupels  have  had  all  volatile  matter  expelled, 
the  assays  are  dropped  into  them,  the  check  in  the  center  one, 
and  the  cupellations  carried  on  in  the  usual  way,  with  feathers. 
After  the  blick,  the  cupels  are  drawn  to  the  front  of  the  muffle 
and  covered  with  extra  cupels.  Sprouted  buttons  must  be  re- 
jected. The  beads  are  now  cleaned,  weighed,  and  rolled  out, 
parted  in  flasks,  with  the  acids  as  described  for  the  preliminary 
assay,  and  the  gold  weighed. 

The  difference  between  the  silver  actually  used  in  the  check 
and  that  found  by  assay  is  the  correction  to  be  added  to  the 
mean  silver  result  of  the  two  bullion  assays  made,  which  should 
not  differ  by  more  than  a  millieme  (0.5  point  fineness).  This 
correction  may  be  plus  or  minus,  according  to  the  amount  of 
copper  in  the  bullion;  for  with  much  copper,  some  of  this  may 
be  retained  by  the  silver  and  give  rise  to  a  minus  correction. 
The  gold  is  corrected  in  the  same  way  as  the  silver.  The  sub- 
traction from  500  of  the  sum  of  the  corrected  silver  and  gold 
gives  the  amount  of  base  metal.  The  individual  results  obtained, 
multiplied  by  two,  express  the  assay  results  in  fineness. 

When  metals  of  the  platinum  group  are  present,  the  method 
must  be  modified  as  outlined,  in  Chapter  XIII,  for  the  assay  of 
platinum,  etc. 

Wei  Methods:  Gay-Lussac  or  Mint  Method.  —  This  method  is 
a  most  accurate  one  and  is  based  on  the  complete  precipitation 
of  Ag  as  AgCl  in  a  nitric  acid  solution  by  means  of  sodium  chloride. 
The  reaction  is  as  follows: 

AgNO3  +  NaCl  =  AgCl  +  NaNO3 
i  part  Ag  =  0.54207  NaCl 

The  standard  solution  of  NaCl  usually  employed  is  of  such 
strength  that  100  c.c.  precipitates  i  gram  of  Ag,  so  that  5.4207 
grams  of  c.p.  NaCl  are  dissolved  per  liter  of  distilled  water  to 
give  the  standard  solution.  This  solution  can  also  be  made  up 
by  using  a  saturated  salt  solution  at  60°  F.,  and  then  adding 
2.07  parts  of  this  to  97.93  parts  of  distilled  water.  The  last 


THE  ASSAY   OF   BULLION 


139 


method  of  obtaining  the  solution  is  not  as  good  as  the  first,  owing 
to  the  difficulty  of  obtaining  the  precise  temperature  of  60°  F. 
and  keeping  it  there.  Aside  from  the  standard  solution  mentioned, 
there  is  required  another  of  one-tenth  its  strength  (obtained  by 
taking  i  part  of  the  standard  NaCl  solution  and  adding  to  it 
9  parts  of  distilled  water),  and  an  acidulated  solution  of  AgNO3, 
obtained  by  dissolving  i  gram  of  proof  silver  in  1 5  c.c.  of  HNO3, 
1.26  sp.  gr.,  and  diluting  with  distilled  water  to  1000  c.c.  It 
follows  from  the  above  that  i  c.c.  of  the  one-tenth  solution  will 
just  precipitate  the  Ag  in  i  c.c.  of  the  acidulated  silver  nitrate 
solution. 

The  standard  NaCl  solution  is  termed  the  "normal  salt" 
solution  in  the  assay,  although  not  properly  so;  the  weak  solution 
is  termed  the  "decimal  salt  solution,"  and  the  silver  nitrate 
solution  the  "decimal  silver"  solution. 

Standardising  Solutions.  —  The  apparatus  required  is : 

1.  A  large  bottle  or  carboy,  containing  the  normal  salt  solu- 
tion, placed  on  an  elevated  shelf  so  that  the  solution  may  be 
siphoned  by  means  of  glass  tubing  and  rubber  hose  to  the  main 
loo-c.c.  pipette. 

2.  Liter  bottles  containing  respectively  the  decimal  salt  and 
the  decimal  silver  solutions. 

3.  An  accurate  loo-c.c.  pipette,  clamped  to  a  suitable  stand, 
and  provided  at  the  top  with  a  glass  overflow-cup  containing  a 
moistened  sponge  to  catch  the  overflow  of  the  normal  salt  solu- 
tion. 

4.  Two  small  graduated  lo-c.c.  pipettes,  one  for  the  decimal 
salt  and  one  for  the  decimal  silver  solution.     Burettes  may  be 
used  in  place  of  these. 

5.  A  number  of  strong  8-  to  i2-oz.  bottles,  similar  to  reagent 
bottles,  provided  with  rubber  corks. 

The  standardizing  of  solutions  is  carried  out  as  follows:  Two 
portions  of  exactly  1002  mgs.  proof  silver  are  dissolved  in  15  c.c. 
of  1.26  sp.  gr.  HNO3,  the  nitrous  fumes  are  removed  by  boiling, 
the  solution  is  transferred  to  the  titration  bottles  and  water 
added  to  bring  up  the  amount  of  solution  to  125  c.c.  The  loo-c.c. 
pipette  is  then  filled  with  normal  salt  solution  to  the  mark,  after 
washing  out  with  salt  solution  to  prevent  dilution.  The  filling 
is  done  by  fastening  the  siphon  hose  to  the  bottom  of  the  pipette, 


140  A   MANUAL  OF   FIRE  ASSAYING 

opening  the  clamp  on  the  hose,  and  letting  the  pipette  fill  with  a 
little  overflow.  The  solution  is  then  shut  off  by  clamping  the 
hose,  a  finger  placed  on  the  top  opening  of  the  pipette  to  prevent 
the  solution  running  out,  and  the  hose  removed.  The  pipette 
is  then  permitted  to  drain  to  the  loo-c.c.  mark,  and  the  solution 
held  there  by  closing  the  top  of  the  pipette  with  the  finger.  The 
bottle  containing  the  dissolved  proof  silver  is  then  placed  under 
the  pipette  and  the  normal  salt  solution  permitted  to  completely 
drain  into  it.  The  bottle  is  then  violently  shaken  for  three  or 
four  minutes,  either  by  hand  or  a  mechanical  agitator,  and  the 
AgCl  allowed  to  settle,  leaving  the  supernatant  liquid  clear.  If 
the  normal  solution  is  made  up  correctly,  it  will  have  precipitated 
just  1000  mgs.  of  silver,  leaving  2  mgs.  unprecipitated.  One  c.c. 
of  decimal  salt  solution  is  now  added  to  the  bottle  by  means  of 
one  of  the  lo-c.c.  pipettes  or  a  burette,  which,  if  the  solution 
still  contains  Ag  unprecipitated,  gives  rise  to  a  white  cloud  of 
AgCl.  The  bottle  is  again  shaken,  the  precipitate  allowed  to 
settle,  and  another  c.c.  of  decimal  salt  solution  added.  If  this 
fails  to  give  a  precipitate,  then  100.  i.  c.c.  of  normal  salt  solution 
are  equivalent  to  1002  mgs.  of  silver  (i  c.c.  of  decimal  salt  solution 
=  o.i  c.c.  normal  salt  solution).  If  the  second  addition  of 
decimal  salt  solution  gives  a  precipitate,  the  shaking  and  settling 
are  repeated,  and  a  third  and  fourth,  etc.,  addition  made,  until  no 
further  cloud  appears.  The  assayer  soon  learns  to  judge  by  the 
density  of  the  cloud  whether  only  part  of  the  c.c.  has  been  used 
up.  In  this  way  he  should  be  able  to  judge  to  the  fourth  of  a 
c.c.  or  the  half  of  a  millieme.  If  the  first  addition  of  decimal 
salt  solution  fails  to  give  a  precipitate,  the  normal  solution  con- 
tains an  excess  of  salt,  and  2  c.c.  of  decimal  silver  solution  are 
now  added,  one  of  which  neutralizes  or  precipitates  the  i  c.c.  of 
decimal  salt  solution  added,  the  other  acting  on  the  excess  of  salt 
in  the  solution.  The  decimal  silver  solution  is  added  until  no 
further  cloud  appears,  in  the  same  way  as  described  for  the 
decimal  salt  solution.  In  this  way  the  exact  strength  of  the 
normal  salt  solution  is  determined  in  duplicate.  If  it  is  incorrect 
to  the  extent  of  more  than  2  points  fineness  either  way  (i.e.,  either 
strong  or  weak),  it  is  corrected  by  the  addition  of  either  water 
or  salt,  and  restandardized,  and,  when  correct,  a  new  decimal 
salt  solution  made  up  from  it.  Its  strength  is  finally  recorded 


THE  ASSAY   OF   BULLION 


141 


on  the  bottle  as  follows:  100  c.c.  =  1000  mgs.  Ag,  or  whatever  it 
may  actually  be. 

The  Assay.  —  It  is  evident  from  the  preceding  that  the  amount 
of  bullion  to  be  taken  for  assay  must  contain  as  nearly  as  possible 
1000  mgs.  Ag  in  order  to  make  the  titration  with  solution  as 
short  as  possible,  and  avoid  undue  additions  of  the  decimal 
solutions.  For  this  reason  the  bullion  on  which  the  silver  deter- 
mination is  to  be  made  is  first  assayed  by  the  cupellation  method, 
or  at  least  a  preliminary  assay,  described  under  this  method,  is 
made,  and  from  these  data  the  amount  of  bullion  containing 
moo  mgs.  of  silver  calculated.  For  instance,  suppose  the  cupella- 
tion method  shows  the  bullion  to  be  900  fine  in  silver,  then 

900         :  1000  ::     1000      :  x. 

fineness     :     amt.  of  bullion    ::     silver     :    amt.  of  bullion. 

or  i  in. ii  mgs.  bullion  contains  1000  mgs.  Ag.  This  amount  of 
bullion  is  then  weighed  out  in  duplicate  and  dissolved  in  acid, 
placed  in  titration  bottles,  as  described  above,  under  "Standardi- 
zation of  Solutions/'  and  titrated. 

The  calculation  for  fineness  is  as  follows :  Suppose  the  strength 
of  the  normal  solution  is  100  c.c.  =  1001  mgs.  Ag,  and  that  99.8  c.c. 
of  normal  solution  were  used  in  the  titration  (100  c.c.  normal 
salt,  and  2  c.c.  decimal  silver);  then 

100     :     1001      ::    99.8     :         x 

the  x,  or  amount  of  silver  in  bullion,  equaling  998.99  mgs.;  and 
the  fineness  is 

mi.  ii      :    998.99     ::     1000      :        y 

the  y,  or  fineness,  equaling  899.1. 

The  only  metal  interfering  with  the  salt  titration  is  mercury, 
which  will  be  precipitated  by  the  NaCl  as  Hg2Q2;  the  addition  of 
20  c.c.  sodium  acetate  and  a  little  free  acetic  acid  to  the  assay 
will  prevent  the  precipitation  of  the  mercury.  Mercury  can  be 
detected  in  the  titration  if  the  AgCl  has  not  turned  dark  as  the 
result  of  exposure  to  sunlight.  Mercury  will  be  found  sometimes 
in  mill  bullions  which  have  been  retorted  at  too  low  a  temperature. 
The  assay  and  standardization  of  the  solution  should  be  carried 
out  where  there  is  no  sun,  and  where  light  is  not  too  strong. 


142  A   MANUAL  OF   FIRE  ASSAYING 

The  Assay  of  Gold  Bullion.  — i  Sampling.  —  Bullion  bars 
and  retort  sponge,  as  shipped  to  the  United  States  assay  offices 
and  mints,  is  remelted  into  bars  to  make  the  deposit  uniform. 
These  are  sampled  by  taking  chips  from  diagonally  opposite 
corners,  each  of  which  is  rolled  into  a  fillet  and  assayed  by  different 
assayers,  who  are  required  to  check  with  each  other  within  narrow 
limits;  if  they  do  not,  the  bar  is  remelted,  stirred  thoroughly,  and 
recast;  then  sampled  again  and  assayed.  If  base  bullion,  or  one 
which  liquates  seriously  on  cooling,  is  to  be  assayed,  dip-samples 
are  taken  from  the  molten  bullion  by  means  of  a  small  graphite 
ladle,  and  the  sample  granulated  in  warm  water.  Silver  bullion 
is  sampled  in  the  same  manner. 

2.  Preliminary  Assay.  —  This  is  made  in  the  way  described 
for  silver  bullion,  except  that  in  the  assay  of  gold  bullion  no 
determination  of  silver  is  made  by  cupellation;  but  if  this  is  to 
be  determined,  the  mint  wet  method  is  used.  Experienced 
assayers  can  judge  the  approximate  fineness  of  gold  bullion  by 
the  color,  and  add  the  proper  amount  of  silver  necessary  to 
insure  parting.  In  the  San  Francisco  mint,  2  parts  of  Ag  to  i 
of  Au  are  used.1  The  British  royal  mint  formerly  used  2.75 
parts  of  Ag.  to  i  of  Au,2  but  now  uses  2  to  i .  More  than  3  parts 
Ag  to  i  of  Au  should  not  be  used,  otherwise  the  "cornet"  of 
gold  is  apt  to  break  up.  With  less  than  2  parts  of  Ag,  too  much 
Ag  is  retained,  although  with  continued  boiling  1.75  parts  Ag 
will  part  Au  from  Ag.3  For  the  preliminary  assay,  500  mgs. 
(1000  milliemes)  are  weighed  out,  silver  added  according  to 
judgment  to  bring  the  ratio  of  silver  to  gold  to  2  or  2.5  (allowing 
for  silver  in  the  alloy),  and  the  bullion  and  silver  wrapped  in 
10  grams  sheet  lead  and  cupeled  at  775°  C. 

The  resultant  bead  is  cleaned,  weighed,  flattened  and  rolled 
out  in  jeweler's  rolls  to  a  fillet  of  the  approximate  thickness  of  a 
visiting  card.  If  some  copper  is  present  in  the  bullion,  enough 
is  retained  by  the  gold  bead  to  toughen  it,  and  it  can  be  easily 
rolled  without  cracking,  if,  between  reductions  by  the  rolls,  the 
fillet  is  annealed  at  a  dull-red  heat.  The  presence  of  copper  in 

1  John  W.  Pack,  "Assaying  of  Gold  and  Silver  in  U.  S.  Mint,"  in  "Min.  and 
Sci.  Press,"  Nov.  14,  1903. 

2  Rose,  in  "Eng.   &  Min.  Journ.,"  LXXX,  p.  492. 

3  Rose,  "Metallurgy  of  Gold,"  p.  493. 


THE  ASSAY   OF   BULLION  143 

the  button  aids  in  the  total  removal  of  lead  during  the  cupella- 
tion.1 

The  fillet  is  then   again  annealed  and  rolled  into  a  spiral, 
called  a  "cornet,"  and  parted  in  a  parting  flask.     This  is  filled 


FIG.  43. —  JEWELER'S  ROLLS 

with  30  c.c.  of  HNO3  sp.  gr.  1.20,  free  from  Cl,  H2SO4,  H2SO3, 
or  any  sulphide,  and  heated  to  boiling  (or  at  least  90°  C.)  for  20 
minutes.  The  acid  is  then  decanted  off,  and  the  cornet  washed 
carefully  several  times  with  hot  distilled  water  by  decantation. 
Then  30  c.c.  of  boiling  nitric  acid,  sp.  gr.  1.30,  are  added  to  the 

1  Rose,  " Refining  Gold  Bullion,"  in  Trans.  I.  M.  M.,  April  13,  1905. 


144  A   MANUAL  OF   FIRE  ASSAYING 

flask,  and  the  cornet  boiled  again  for  20  minutes,  after  which  the 
acid  is  decanted,  and  the  washing  with  hot  water  repeated. 
During  the  boiling,  a  parched  pea  added  to  the  flask  prevents 
bumping.  The  flask  is  now  filled  to  the  very  top  with  cold 
distilled  water,  a  suitable  sized  porcelain  parting-cup  placed  over 
the  mouth,  fitting  reasonably  tight,  and  the  flask  inverted.  The 
cornet  will  settle  into  the  parting-cup,  and  the  flask  is  then  gently 
tipped  to  permit  the  water  to  escape,  the  water  is  decanted  from 
the  parting-cup,  and  the  cornet  gently  dried.  When  dry,  the 
cornet  is  transferred  to  a  clay  annealing  cup,  the  cover  is  put 
on,  and  the  cup  is  placed  in  the  muffle,  and  the  cornet  annealed 
at  a  full-red  heat.  It  is  then  weighed.  The  weight  of  the  gold 
plus  that  of  the  added  silver,  subtracted  from  the  weight  of  the 
cupeled  bead,  gives  the  approximate  amount  of  silver  in  the 
assay.  This  added  to  the  weight  of  the  gold  and  subtracted 
from  500  mgs.  (the  weight  of  bullion  taken)  gives  the  approximate 
amount  of  base  metal.  If  the  amount  of  silver  added  to  part 
the  gold  has  raised  the  ratio  of  Ag  to  Au  over  3  to  i,  the  gold 
will  probably  have  broken  up,  or  at  least  parts  will  have  broken 
from  the  edges  of  the  cornet;  care  must,  in  this  case,  be  taken  to 
collect  all  of  it  in  the  washing.  If  the  results  show  that  the 
ratio  of  Ag  to  Au  has  been  less  than  2  to  i,  the  cornet  must  be 
recupeled  with  2.5  parts  Ag  and  parted  as  described. 

The  Assay.  —  The  final  assay  is  made  up  from  data  obtained 
in  the  preliminary  assay.  Duplicates  on  1000  milliemes  are  run, 
with  a  check  assay  made  up  in  composition  as  near  to  that  of 
the  bullion  as  possible,  as  described  for  the  cupellation  assay  of 
silver.  In  making  up  the  check,  proof  gold  and  proof  silver  are 
used,  and  c.p.  copper  foil.  The  United  States  mints  use  various 
proof  alloys  in  the  making  up  of  check  assays.  For  the  assay  of 
fine  gold  bars  (990  fineness  and  above),  a  proof  alloy  of  1000 
gold,  2000  silver,  and  30  parts  copper  is  used.  For  coin  metal 
(900  parts  fine),  a  proof  alloy  of  gold  900  parts,  silver  1800  parts, 
copper  100  parts  is  used.  For  the  determination  of  base  metal 
(the  difference  between  the  gold  and  silver,  and  the  500  mgs. 
taken  for  assay),  a  proof  alloy  of  gold  900  parts,  silver  90 
parts,  copper  10  parts  is  used.1  In  this  last  the  gold  need  not 
be  proof  gold,  but  may  be  remelted  cornets.  It  is  to  be  noted 

1  John  Pack,  ibid. 


THE  ASSAY   OF   BULLION  145 

that  these  proof  alloys  are  made  up  on  the  assumption  that  2 
parts  of  Ag  to  I  of  gold  are  used  in  parting.  The  British  mint 
uses  a  proof  alloy,  or  trial  plate,  916.6  fine  in  gold. 

For  the  assay  of  crude  gold  bullion,  i.e.,  mill  bullion,  the 
proof  alloy  for  fine  gold  bars  is  generally  used. 

The  amount  of  lead  used  in  the  cupellation  is  as  follows : : 

TABLE   XXXIII.  — LEAD    RATIO   IN   CUPELLATION 


AMOUNT  or  GOLD  PER    i 

AMOUNT  OF  LEAD 
1000  PARTS 


Milliemes  Grams 


i  RATIO  OF  LEAD  TO  COPPER 
(BASE  METAL  PRESENT) 


916.6 

8 

96  to  i 

366 

9-1-5 

64  to  i 

770 

14-75 

64  to  i 

666 

16.00 

48  to  i 

546 

17-5° 

38  to  i 

333 

18.0 

27  to  i 

To  the  duplicates  of  the  1000  milliemes  of  bullion,  the  proper 
amount  of  Ag  is  added,  to  bring  the  ratio  of  Ag  to  Au  to  2  to  i, 
and  then  they  are  wrapped  in  the  proper  amount  of  c.p.  sheet 
lead.  The  check  is  made  up  as  indicated  by  the  preliminary 
assay,  and  the  three  assays  cupeled  as  described  for  the  assay 
of  silver  bullion.  The  three  beads  are  then  treated  and  parted, 
as  described  for  the  preliminary  assay.  The  two  bullion  assays 
should  not  differ  by  more  than  0.25  part  of  a  millieme.  The 
correction  as  indicated  by  the  check  should  then  be  applied, 
whether  this  be  plus  or  minus.  The  difference  between  the  fine 
gold  in  the  check  and  that  obtained  by  the  assay  of  the  check  is 
the  surcharge,  which  is  more  definitely  defined  in  Chapter  XI, 
on  "Errors  in  the  Assay  for  Gold  and  Silver."  This  surcharge 
will  usually  amount  to  about  o  for  a  bullion  of  about  700  to  800 
fine;  above  that  there  will  be  a  "plus  surcharge/'  and  below  that 
a  "minus  surcharge."  The  plus  surcharge  will  be  added  and  the 
minus  surcharge  subtracted. 

The  Preparation  of  Proof  Gold.  —  This  is  prepared  by  dissolving 

1  Rose,  "Metallurgy  of  Gold,"  1902,  p.  494. 


146  A  MANUAL  OF   FIRE  ASSAYING 

practically  pure  gold  (cornets)  in  nitro-hydrochloric  acid,  per- 
mitting the  solution,  after  some  dilution,  to  stand  for  four  days 
to  allow  AgCl  to  settle  out.  It  is  then  decanted  very  carefully 
by  siphoning.  The  gold  chloride  solution  is  then  evaporated 
almost  to  dryness,  taken  up  with  plenty  of  distilled  water,  a  few 
c.c.  of  NaBr  solution  added,  allowed  to  stand  for  some  days,  and 
again  decanted  by  siphoning,  after  which  operation  it  is  slowly 
dropped  from  a  burette  into  a  beaker  containing  c.p.  aluminum 
foil.  When  precipitation  is  complete,  HC1  is  added  to  dissolve 
the  excess  of  Al,  and  the  residual  gold  is  washed  thoroughly  with 
water  by  decantation,  and  then  dried  and  melted  into  a  bead  in 
a  fresh  cupel  (but  not  cupeled  with  Pb).  The  gold  is  then 
rolled  into  a  thin  strip  for  use.1 

Proof  silver  is  prepared  by  dissolving  c.p.  silver  foil  in  HNO3, 
and  then  precipitating  with  HC1  after  filtering.  The  AgCl  is 
thoroughly  washed  with  diluted  HC1  and  converted  into  metallic 
silver  by  Al  in  the  presence  of  HC1,  all  Al  being  dissolved  out. 
The  washed  silver  is  then  fused  in  a  clean  cupel,  and  rolled  into 
strips.2 

1  Consult  also  Rose,  "Metallurgy  of  Gold,"  p.  12,  and  Pack,  ibid. 

2  John  Pack,  ibid. 


XIII 


THE  ASSAY  OF  ORES  AND  ALLOYS  CONTAINING 
PLATINUM,    IRIDIUM,   GOLD,   SILVER,   ETC. 

MATERIALS  containing  some  of  the  above  elements  are  pre- 
sented to  the  assayer  for  determination  in  the  shape  of  sands 
containing  chiefly  platinum,  alloys  and  jeweler's  sweeps,  and, 
more  rarely,  ores  containing  platinum  in  the  form  of  the  mineral 
sperrylite,  etc. 

The  assay  for  platinum  and  associated  metals  is  a  difficult 
one,  due  to  the  fact  that  in  the  parting  of  the  precious  metal 
beads  by  acids,  complex  reactions  take  place,  by  which  platinum, 
palladium,  silver,  etc.,  both  go  into  solution  and  are  retained  in 
the  residue,  unless  certain  well  established  ratios  of  metals  present 
are  observed  and  the  parting  operation  repeated  several  times. 
The  alloys  of  platinum  and  silver  have  been  most  thoroughly 
investigated  in  this  connection.1  When  the  alloy  is  more  com- 
plex, i.e.,  contains  also  gold,  palladium,  iridium,  rhodium,  etc., 
the  difficulties  of  the  assay  are  increased;  the  data  at  present 
available  are  meager. 

Platinum  nuggets  from  the  Urals  contain:2  Pt,  60  to  86.5  per 
cent.;  Fe,  up  to  19.5  per  cent.;  Ir,  up  to  5  per  cent.;  Rh,  up  to 
4  per  cent.;  Pd,  up  to  2  per  cent.;  also  Os,  Ru,  Cu,  Au,  and  iri- 
dosmium. 

When  material  containing  Au,  Ag,  Pt,  Pd,  Ir,  Rh,  Ru,  Os, 
and  IrOs  is  fused  by  the  crucible  assay  or  melted  with  lead,  the 
Au,  Ag,  Pt,  Pd,  Ir,  Rh,  IrOs  are  collected  by  the  lead  and  the 
Ru,  and  Os  only  partially  so.  If  the  resultant  lead  button  is 
cupeled,  the  final  bead  will  contain  the  Au,  Ag,3  Pt,  Pd,  Ir,  Rh, 

1  Thompson  and  Miller,  in   "Journ,  Am.  Chem.  Soc.,"  XXVIII,  p.   1115. 
See  this  paper  for  other  references. 

2  Kemp,  in  "Eng.  and  Min.  Journ.,"  LXXIII,  p.  513  (Notes  on  Platinum 
and  Associated  Metals). 

3  Exclusive  of  losses  by  absorption  and  volatilization. 


148  A   MANUAL  OF   FIRE  ASSAYING 

IrOs,  and  a  comparatively  small  portion  of  the  Os  and  Ru,  the 
most  of  these  two  metals  being  lost  by  oxidation.  The  presence 
of  any  considerable  amounts  of  Os  and  Ru  in  the  lead  button, 
owing  to  the  fact  that  they  will  not  alloy  readily,  causes  them  to 
appear  as  a  black  scum  or  as  spots  on  the  bead,  near  the  end  of 
the  cupellation.  The  presence  of  the  platinum  group  of  metals, 
raising  the  melting-point  of  the  gold-silver  alloy,  renders  neces- 
sary a  high  temperature  of  cupellation  in  order  to  remove  lead. 
Even  then,  when  the  ratio  of  Ag  to  Pt,  etc.,  is  less  than  5  to  i, 
lead  will  be  retained  in  varying  proportions  at  the  cupellation 
temperature  of  gold  bullion.  To  get  rid  of  the  lead,  the  propor- 
tion should  be  10  to  i.1  The  following  points  on  the  first  cupella- 
tion of  the  lead  buttons,  resulting  from  the  assay  of  material 
containing  Pt,  etc.,  will  give  the  assayer  an  idea  of  what  is  present. 

When  Pt  alone,  or  with  very  little  silver,  is  present,  the  bead 
from  the  cupellation  (at  a  comparatively  high  temperature)  is 
rough,  dull  gray,  flat,  and  contains  lead. 

If  more  silver  is  present,  but  less  than  2  parts  of  Ag  to  i  of 
Pt,  the  beads  are  rough,  flat,  and  have  a  crystalline  surface. 

If  more  than  2  parts  of  Ag  are  present  and  not  more  than  15, 
the  bead  approaches  more  nearly  the  appearance  of  a  normal 
silver  bead,  but  has  a  more  steely  appearance  and  is  flatter  in 
proportion  to  the  Pt,  etc.,  contained. 

Beads  containing  more  platinum  than  i  in  16  will  not  blick 
or  flash.2 

The  effect  on  the  appearance  of  the  bead  of  Pd,  Rh,  Ir  is  similar 
to  that  of  Pt,  but  not  identical. 

Owing  to  the  difficulty  in  alloying  iridium,  this,  when  present, 
is  apt  to  be  found  at  the  bottom  of  the  bead,  in  the  shape  of 
fine  black  crystalline  particles.3 

The  Action  of  Acid  on  the  Alloy  Beads.  —  A  great  deal  of 
literature  exists  on  this  point;  but  most  of  it  is  very  conflicting; 
some  facts,  however,  have  been  definitely  established. 

Nitric  Acid.  —  In  an  alloy  of  Pt  and  Ag  treated  by  HNO3, 
platinum  goes  into  solution  in  various  proportions,  depending 

1  Sharwood,   "Cupellation  on  Platinum  Alloys,  containing  Ag  and  Au,"  in 
"Chem.  Ind.,"  Vol.  XXIII,  No.  8. 

2  Schiffner,  in  "Min.  Ind.,"  VIII,  p.  397. 

3  Rose,  " Metallurgy  of  Gold,"  p.  $^  46 1 


ASSAY   OF  ORES,    PLATINUM,    IRIDIUM,    ETC.  149 

on  the  ratio  of  Ag  to  Pt,  and  probably  to  some  extent  on  the 
strength  of  acid.  It  has  been  stated  that  when  the  ratio  of 
Ag  to  Pt  is  12  or  15  to  i,  this  solution  of  Pt  is  complete  in  one 
treatment,  but  this  has  been  disproved  by  later  investigation.1 
In  order  to  accomplish  the  solution  of  Pt,  the  acid  treatment 
must  be  repeated  at  least  once  or  twice,  with  a  possible  recupel- 
lation  of  the  residue  with  silver  before  the  second  treatment. 
It  is  even  then  doubtful  if  all  of  the  Pt  can  be  dissolved.  The 
Pt  goes  into  solution  in  the  nitric  acid  in  colloidal  form,  giving 
a  brown  to  blackish  color  to  the  solution.  The  reason  for  this 
seems  to  be  the  presence  of  platinum-silver  compounds  in  the 
alloy,  some  of  which  are  insoluble,  or  only  partially  soluble,  in 
HNO3.  When  gold  is  present  in  the  silver-platinum  alloy,  the 
solubility  of  the  Pt  seems  to  be  decreased,2  unless  the  ratio  of 
Pt  to  Au  to  Ag  is  1:2:  i5,3  when  most,  but  not  all,  of  the  Pt 
and  all  the  Ag  go  into  solution.  Palladium  goes  into  solution 
with  nitric  acid  when  at  least  3  parts  of  Ag  to  I  of  Pd  are  present,4 
yielding  an  orange-colored  solution;  but  double  parting  is  neces- 
sary to  insure  complete  solution.  (This  point  is  not  sufficiently 
established.5)  The  orange-colored  solution  indicates  colloidal 
palladium. 

Iridium  and  Rhodium.  —  Iridium  present  in  the  beads  is 
unacted  upon  by  HNO3  and  remains  with  the  gold.6  Rhodium 
is  slightly  dissolved,  but  most  of  it  remains  with  the  gold.  Iridos- 
mium  is  not  dissolved.  Osmium  is  dissolved.  Ruthenium  is 
not  dissolved. 

Sulphuric  Acid.  —  Platinum,  alloyed  with  silver  and  gold, 
can  be  separated  from  the  silver  and  remains  with  the  gold,  if 
concentrated  sulphuric  acid  is  used  in  parting.  In  order  to 
insure  thorough  parting,  at  least  10  parts  of  silver  to  i  part  Pt 
and  gold  should  be  present,  and  double  parting  resorted  to, 
otherwise  silver  will  remain  with  the  residue.7  The  parting  in 
H2SO4  leaves  the  Pt  and  gold  in  a  very  fine  state  of  division  (but 
not  as  a  colloid),  some  of  which  is  very  apt  to  be  lost  in  decanting, 

1  Thompson  and  Miller,  in  "  Journ.  Am.  Chem.  Soc.,"  XXVIII,  p.  115. 

2  Sharwood,  ibid. 

3  Lodge,  "Notes  on  Assaying,"  p.  215. 

4  Rose,  " Metallurgy  of  Gold,"  p.  >^.     461        6  Rose,  ibid. 

5  Lodge,  "Notes  on  Assaying,"  pp.  218,  219.        *  Thompson  and  Miller,  ibid. 


150  A  MANUAL  OF   FIRE  ASSAYING 

so  that  it  is  best  to  separate  by  filtering  through  an  ashless  filter. 
It  is  also  to  be  noted  that  lead  may  be  present  in  consequence  of 
too  low  a  cupellation  temperature,  in  which  case  residue  should 
be  treated  with  ammonium  acetate,  to  remove  lead  sulphate. 

Palladium.  —  In  parting  with  H2SO4  this  goes  into  solution 
with  the  silver,  giving  an  orange-colored  solution.  Whether  this 
solution  is  complete,  has  not  as  yet  been  demonstrated.1 

Ir,  IrOs,  Rh,  and  Os  and  Ru  in  the  bead  are  not  dissolved. 

Nitro-Hydrochloric  Acid.  —  From  the  residue  of  the  sulphuric 
acid  parting,  the  Pt,  Au,  and  any  Pd  left  is  dissolved  by  dilute 
aqua  regia,  i  to  5,  leaving  Ir,  IrOs,  and  Rh,  and  some  Ru  and  Os, 
if  present.  This  last  residue,  treated  with  strong  aqua  regia, 
removes  Ir,  leaving  iridosmium  and  rhodium  as  a  final  residue. 

Methods  of  Assay.  —  i.  Ores.  —  Rich  ores,  carrying  Pt,  etc., 
in  grains,  present  difficulty  in  sampling,  inherent  to  any  ore 
containing  "metallics."  It  is  best  to  take  from  30  to  50  grams 
of  the  sample  and  fuse  it  with  6  times  its  weight  of  lead  in  a 
crucible,  fluxing  the  gangue.  The  lead  is  poured,  and  after 
cooling  the  slag  is  detached  carefully,  the  lead  platinum  alloy 
being  brittle,  weighed  and  remelted  under  charcoal  in  order  to 
insure  a  uniform  alloy,  and  then  granulated  as  fine  as  possible 
by  pouring  into  a  large  volume  of  cold  water  from  a  considerable 
hight.  The  resultant  sample  is  then  dried  and  is  ready  for 
assay.  An  amount  containing  approximately  200  mgs.  Pt.  is 
weighed  out  and  scorified  with  50  grams  Pb  into  a  2o-gram 
button. 

If,  in  the  low-grade  ores,  the  Pt,  etc.,  is  present  as  grains,  a 
weighed  quantity  is  concentrated  by  panning  and  the  concen- 
trates scorified  with  20  to  25  times  their  weight  of  test  lead,  and 
the  button  treated  according  to  method  No.  i  or  2,  as  below. 
If  the  ore  contains  the  rare  metal  in  other  form,  crucible  fusions 
are  made  on  i  assay  ton,  as  with  gold  and  silver  ores,  and  if  very 
low  grade,  the  buttons  from  4  to  5  fusions  are  scorified  into  one 
button,  final  duplicates  being  made  as  usual.  The  lead  buttons 
are  treated  as  below. 

2.  Alloys.  —  An  amount  of  drillings  or  filings  (representing 
a  true  sample  of  the  alloy),  containing,  if  possible,  not  to  exceed 
200  mgs.  of  Pt.  etc.,  is  weighed  out  and  scorified,  with  80  grams 

1  Lodge  holds  the  contrary,  p.  219. 


ASSAY   OF  ORES,   PLATINUM,    IRIDIUM,    ETC.  151 

of  test  lead,  to  a  button  of  about   18  to  20  grams.     The  lead 
buttons  are  treated  as  outlined  below. 

First  Method.  —  The  lead  button  obtained  by  any  of  the 
foregoing  methods  is  cupeled  at  a  temperature  of  at  least  800°  C., 
or,  better,  850°  C,  and  the  resultant  bead  examined.  If,  from 
the  foregoing  description  of  the  appearances  of  a  bead,  it  is 
thought  that  the  ratio  Ag  to  Pt,  Au,  etc.,  is  less  than  10  to  i, 
the  button  is  removed,  the  necessary  silver  added  to  bring  it  up 
to  the  above  ratio,  recupeled  with  5  to  8  grams  of  lead  at  a  tem- 
perature of  800°  C.,  and  weighed.  The  bead  is  then  flattened 
and  rolled  out  into  a  cornet,  if  large  and  not  too  brittle,  and 
parted  with  15  c.c.  H2SO4  concentrated,  boiling  for  15  to  20 
minutes.  The  acid  is  then  decanted  into  a  beaker  and  saved, 
the  residue  re-treated  with  5  c.c.  more  of  acid  for  10  minutes, 
and  the  residue  and  acid  washed  into  the  beaker  containing 
the  previous  acid.  The  acid  is  then  diluted  and  the  residue 
separated  by  filtration  through  a  small  ashless  filter,  and  thor- 
oughly washed  with  hot  water  to  insure  removal  of  Ag2SO4. 
The  filter-paper  is  dried  and  carefully  transferred  to  a  porcelain 
parting-cup  or  an  annealing  cup,  and  the  carbon  burnt  off  in 
the  muffle.  The  annealed  residue  is  brushed  out  on  the  scale 
pan  of  the  bead  balance  and  weighed.  It  consists  of  gold,  plati- 
num, indium,  iridosmium,  rhodium,  and  possibly  osmium  and 
Ru  (if  any  escaped  oxidation  during  the  cupellation),  and  perhaps 
some  palladium.  Its  color  will  be  gray  or  black,  if  the  rare 
metals  are  present  to  any  extent.  If  not,  the  characteristic  gold 
color  will  show.  The  palladium  is  largely  in  the  filtrate.  (It  is 
questionable  how  complete  this  solution  is.1)  If  it  has  been 
unnecessary  to  add  Ag  to  the  cupellation  to  get  the  10  to  i  ratio, 
the  difference  in  weight  between  the  original  bead  and  the  weight 
of  the  residue  represents  the  Ag.  If  silver  had  to  be  added  and 
the  bead  recupeled,  the  weight  of  the  added  silver  plus  that  of 
the  residue,  subtracted  from  the  weight  of  the  recupeled  bead, 
gives  the  silver.  Allowance  must,  however,  be  made  for  con- 
siderable loss  of  silver  as  a  result  of  high  cupellation  temperature. 
If  accurate  silver  results  are  required,  a  duplicate  assay  on  the 
material  must  be  run,  and  the  silver  requisite  to  bring  the  ratio 

1  Ricketts  and  Miller,  in  "Notes  on  Assaying,"  state  that  the  Pd  dissolves 
with  the  Ag. 


i52  A   MANUAL  OF   FIRE  ASSAYING 

up  to  10  to  i  is  added  at  once  to  the  lead  button,  one  cupellation 
only  being  made.  At  the  same  time  this  is  run,  a  check  assay 
is  run  beside  it,  made  up  of  the  same  weight  of  lead,  and  the 
proper  weight  of  silver,  i.e.,  the  amount  added  to  the  first  cupel- 
lation plus  the  amount  approximately  known  to  be  in  the  assay. 
The  loss  in  this  will  give  the  correction  to  be  added  to  the  assay 
for  Ag.  It  may  be  desirable  to  determine  Ag  in  the  wet  way. 
(See  "The  Assay  of  Silver  Bullion.") 

The  residue  is  now  wrapped  in  8  to  10  grams  of  lead  foil  with 
at  least  20  times  its  weight  in  silver  and  cupeled  again  at  a 
high  temperature.  The  bead,  if  large,  is  rolled  out  and  heated 
to  boiling  in  a  mattrass  or  flask  for  20  minutes  with  HNO3, 
sp.  gr.  i. 20,  after  which  the  acid  is  decanted  into  a  beaker,  and 
the  treatment  repeated  with  HNO3  of  1.26  sp.  gr.  The  residue, 
if  finely  divided,  should  now  be  filtered  through  an  ashless  filter 
and  washed  as  already  described.  If  not,  the  filtrate  can  be 
decanted  and  the  residue  washed.  The  residue  consists  of  Au, 
Ir  and  iridosmium,  and  some  Rh  and  Ru.  If  there  is  any  sus- 
picion that  any  platinum,  etc.,  remains,  the  residue  must  be 
re-treated  with  acid  until  of  constant  weight.  The  platinum  is 
in  the  filtrate,  which  will  be  colored  brown  or  black. 

The  difference  between  the  weights  of  the  first  and  second 
residues  is  platinum,  the  result  possibly  being  somewhat  high  if 
palladium  is  present  in  the  material  assayed.  The  second  residue 
is  now  warmed  in  a  mattrass  with  dilute  aqua  regia  1  (i  to  5) 
for  15  minutes.  This  dissolves  the  gold,  some  of  the  Ru  and 
very  little  Rh,  leaving  the  Ir,  iridosmium  and  Rh,  with  some 
Ru.  The  residue  is  either  filtered  or  decanted,  as  necessary, 
dried,  annealed,  and  weighed.  The  difference  in  weight  between 
the  second  and  third  residues  represents  gold,  somewhat  high,  if 
the  Ru  has  partly  escaped  oxidation  and  volatilization  during 
cupellation.  The  gold  can  be  recovered  by  precipitation  with 
oxalic  acid,  as  described  in  the  second  method. 

If  the  third  residue  is  treated  with  strong  aqua  regia,  and 
boiled,  it  dissolves  out  the  iridium,  leaving  as  a  residue  the 
iridosmium  and  most  of  the  Rh.  This  is  dried,  annealed,  and 
weighed,  the  difference  in  weight  between  the  third  and  fourth 

1  Concentrated  aqua  regia  is  i  part  HNO3,  sp.  gr.  1.42,  and  3  parts  HC1, 
sp.  gr  1.20. 


ASSAY   OF  ORES,    PLATINUM,    IRIDIUM,    ETC.  153 

residues  representing  indium,  and  the  weight  of  the  fourth  residue 
representing  iridosmium  and  Rh.  The  method  determines  Ag, 
Pt,  Au,  Ir,  and  iridosmium  plus  Rh.  The  probable  errors  in 
the  determination  have  been  pointed  out.  Palladium  can  be 
satisfactorily  determined  only  by  wet  analysis. 

Second  Method.1  —  Take  the  lead  button  from  the  ore  or 
alloy  assay,  and  scorify  at  a  high  heat,  with  additional  test  lead, 
if  necessary,  to  a  weight  of  8  to  10  grams.  It  should  contain 
less  than  5  per  cent.  Pt,  etc.,  in  order  to  be  malleable.  Roll  out 
the  button  into  a  long  thin  fillet  and  place  in  a  large  beaker  with 
200  c.c.  of  HNO3,  sp.  gr.  i.o8,2  and  heat  until  all  action  ceases. 
Filter  through  a  small  ashless  filter  and  wash  the  residue  with 
hot  water.  Dry  the  residue  and  paper,  transfer  to  a  large-size 
parting-cup  and  ignite  in  the  muffle,  to  burn  off  the  carbon,  and 
oxidize  any  Pb  not  dissolved.  Then  heat  to  boiling  in  the  cup 
with  HNO3,  i. 08  sp.  gr.,  decant,  wash  thoroughly  with  hot  water, 
dry,  anneal,  and  weigh  the  residue.  This  consists  of  Au,  Pt,  Ir, 
iridosmium,  and  most  of  the  Rh,  as  well  as  the  Ru  and  Os  which 
escaped  oxidation  and  volatilization  during  the  scorification. 
The  filtrate  contains  the  Ag  and  Pd  and  a  little  of  the  Rh. 

Replace  the  residue  in  the  capsule  and  warm  (not  boil)  with 
dilute  aqua  regia  (i  to  5)  for  10  minutes.  This  dissolves  the 
Au  and  Pt.  Decant  the  solution  into  a  small  beaker,  wash  the 
residue,  dry,  anneal,  and  weigh.  The  second  residue  consists  of 
Ir,  IrOs,  Rh,  and  a  little  Os  and  Ru.  This  residue  is  boiled 
with  strong  aqua  regia,  which  dissolves  the  Ir  and  some  Os  and 
Ru,  and  leaves  in  the  third  residue  the  IrOs  and  Rh,  with  a  little 
Os  and  Ru.  This  is  washed,  decanted,  and  weighed  as  before. 
The  filtrate  from  the  treatment  of  the  first  residue,  which  con- 
tains the  gold,  is  evaporated  just  to  dryness,  but  not  baked,  so 
as  to  prevent  reduction  to  gold  chloride,  taken  up  with  distilled 
water  and  a  drop  of  HC1,  and  the  gold  in  it  precipitated  by 
warming  with  crystals  of  oxalic  acid  for  a  half  hour,  filtering, 
and  drying  the  yellow  coherent  precipitate  of  gold.  This  is 
transferred,  filter-paper  and  all,  to  a  piece  of  sheet  lead,  silver 
added  to  the  weight  of  3  times  the  gold  present,  approximately, 
and  cupeled,  the  bead  being  parted  in  HNO3  as  usual  and  the 

1  E.  H.  Miller,  in  "Sch.  Mines  Quart.,"  XVII,  p.  26. 

2  Si  parts  distilled  H2O  to  19  parts  HNO3  cone.  (sp.  gr.  1.42). 


i54  A   MANUAL  OF   FIRE   ASSAYING 

gold  annealed  and  weighed.  The  weight  of  the  gold,  subtracted 
from  the  difference  in  weight  between  the  first  and  second  residues, 
is  the  platinum.  This  last  may  also  be  estimated  by  destroying 
the  oxalic  acid  in  filtrate  from  the  separation  of  gold,  and  pre- 
cipitating as  (NHJPtCV 

It  is  to  be  noted  that,  by  the  assay  as  outlined,  neither  osmium 
nor  ruthenium  can  be  determined,  owing  to  their  volatility  during 
part  of  the  operation;  that  palladium  cannot  be  readily  deter- 
mined, owing  to  its  varying  solubility;  and  that  when  rhodium 
or  the  above  metals  are  present  in  any  appreciable  quantity, 
some  of  the  results  obtained  are  liable  to  error.  Rhodium, 
osmium,  and  ruthenium  are  among  the  rarer  of  the  group,  and 
are  frequently  absent.  The  methods  outlined  will  serve  to 
determine  reasonably  well  platinum,  gold,  silver,  iridium,  and 
iridosmium  plus  rhodium.  When  the  other  elements  of  the 
group  are  present,  wet  methods,  not  within  the  scope  of  this 
book,  must  be  resorted  to. 

In  the  ordinary  assay,  as  carried  out  for  gold  and  silver, 
platinum  and  palladium  may  escape  the  assayer  if  present  in 
only  small  quantities,  for  obvious  reasons.  Parting  in  sulphuric 
acid  is  therefore  necessary  to  determine  whether  they  are  present.2 

1  Crookes,  "Select  Methods." 

2  An  orange-colored  solution  indicates  palladium. 


XIV 


THE  ASSAY  OF  TIN,  MERCURY,  LEAD,  BISMUTH  AND 

ANTIMONY 

THE  assay  of  ores  for  base  metal  by  fusion  is  still  carried  out  in 
practice,  especially  for  lead  and  tin.  The  fire  assay  gives,  not 
the  correct  metal  content,  but  the  yield  obtainable  in  smelting, 
although  in  metallurgic  operations  the  yield  may  be  greater  or 
less.  The  smelter,  therefore,  purchases  lead,  tin,  and  copper 
ores  on  the  basis  of  the  "dry"  or  fire  assay.  The  fire  assay  of 
copper  is  practically  no  longer  in  use,  except  in  part  of  the  Lake 
Superior  district,  on  metallic  copper  concentrates,  and  in  pur- 
chasing copper  ores  the  assay  is  made  by  the  standard  electrolytic 
method,  or  a  volumetric  method,  and  a  percentage  of  from  i  to 
1.5  deducted  to  indicate  dry  assay.  The  usual  deduction  is 
i  .3  per  cent.  Thus  the  dry  assay  of  copper  on  an  ore  is  equivalent 
to  the  percentage  obtained  by  the  electrolytic  method  less  1.3 
per  cent. 

While  wet  methods,  with  a  deduction,  will  in  all  probability 
be  employed  eventually  for  all  lead  ores,  as  it  is  now  for  impure 
lead  ores,  pure  lead  ores  are  still  assayed  by  the  fire  method. 
Tin  ores  are  almost  invariably  assayed  by  the  fire  method,  as  the 
wet  analysis  of  tin  is  long  and  tedious. 

THE  ASSAY  OF  TIN  ORES 

The  fire  assay  of  tin  ores  is  applicable  only  to  those  ores  in 
which  tin  exists  as  cassiterite,  the  oxide  (SnO2).  The  chief 
reasons  for  inaccuracies  in  the  fire  assay  of  tin  are: 

1.  Some  of  the  tin,  reduced  in  the  assay  from  the  oxide,  is 
apt  to  be  volatilized  at  the  temperatures  necessarily  employed. 

2.  Metallic  tin  may  be  slagged  by  alkaline  carbonates  used 
in  some  of  the  methods  of  assay,  forming  stannates. 


156  A  MANUAL  OF   FIRE  ASSAYING 

3.  Foreign  metals  present  in  the  ore  are  apt  to  be  reduced 
and  enter  the  button. 

4.  Sulphides  present  carry  tin  into  the  slag.     If  sulphates 
are  present,  they  are  reduced  to  sulphides. 

5.  Silica  and  silicates,  always  present  in  the  ore,  even  after 
very  careful  concentration,  carry  tin  into  the  slag,  as  silicate, 
while  the  SnO2  passes  through  the  lower  stage  of  oxidation  in 
being  reduced  to  metallic  tin. 

6.  The  cassiterite,  before  reduction,  is  apt  to  combine  with 
basic  fluxes  present  in  the  assay,  and  be  carried  into  the  slag  as 
stannates. 

From  this,  therefore,  it  is  evident  that  the  fire  assay  for  tin 
is  only  approximation,  although  in  many  cases  a  very  close  one. 
If  the  result  on  a  tin  ore  by  the  fire  method  checks  that  of  the 
standard  wet  method  (the  modified  Rose  method  l),  it  is  to  be 
ascribed  to  a  balancing  of  errors,  due  to  the  presence  of  other 
metals  in  the  ore,  which  have  been  reduced  into  the  tin  button. 

Preparation  of  the  Ore  for  Assay.  —  It  is  essential  to  remove 
all  the  gangue  of  the  ore  and  have  for  the  assay  nothing  but  the 
cassiterite,  as  far  as  this  is  possible.  The  ore  is  roughly  crushed 
on  a  buck  board  and  put  through  a  4O-mesh  screen,  crushings 
and  screenings  succeeding  each  other  at  frequent  intervals  in 
order  to  avoid  the  "sliming"  of  the  cassiterite.  If  the  ore  is 
low-grade,  i.e.,  below  2  per  cent.  Sn,  1000  grams  of  the  crushed 
ore  is  weighed  out  and  carefully  panned  in  a  gold  pan,  the  first 
pannings  being  saved  for  repanning.  The  ore  is  concentrated 
just  as  much  as  possible  without  incurring  loss  of  cassiterite. 
The  concentrates  from  the  repanning  of  the  tailings  of  the  first 
treatment  are  added  to  the  main  lot  of  concentrates.  Some  or 
all  of  these  will,  unless  the  ore  is  very  pure,  contain  probably 
garnets,  feldspar,  tourmaline,  magnetite,  zircons,  wolframite, 
columbite,  sulphides,  quartz,  etc.  The  concentrates  are  care- 
fully transferred  to  a  porcelain  dish,  dried,  and  roasted  at  a 
bright-red  heat  in  order  to  decompose  sulphides  and  sulphates. 
While  the  concentrates  are  still  red-hot,  they  are  transferred  into 
a  beaker  containing  water  in  order  to  make  garnet  and  other 
silicates  soluble  (all  except  uvarovite),  and  after  decanting  water, 
treated  with  nitro-hydrochloric  acid  to  remove  most  of  the 

1  Hofman,  "The  Dry  Assay  of  Tin  Ores,"  in  Trans.  A.  I.  M.  E.,  XVIII,  p.  i. 


ASSAY   OF  TIN,   MERCURY,   LEAD   BISMUTH  157 

contaminating  minerals,  except  quartz,  wolframite,  and  some 
garnet.  The  concentrates  are  then  filtered  off  and  dried.  If 
quartz  is  present,  this  can  be  removed  by  transferring  the  filtered 
concentrates  to  a  platinum  dish  and  treating  with  HF.  This, 
however,  will  rarely  be  necessary.  The  concentrates  are  then 
crushed  in  an  agate  mortar  to  pass  a  loo-mesh  screen  and  treated 
as  described  below. 

The  Assay.  —  The  two  best  methods  for  assay  are  the  cyanide 
fusion  and  the  German  method,  with  black  flux  substitute.  Of 
these  two,  the  cyanide  fusion  is  generally  to  be  preferred,  as  any 
minerals  still  left  in  the  cassiterite  have  less  influence  on  the 
assay,  and  the  loss  of  tin  by  volatilization  is  reduced  to  a  mini- 
mum, on  account  of  the  low  temperature  employed. 

The  Cyanide  Fusion.1  —  It  is  essential  to  use  only  the  purest 
cyanide  obtainable  —  the  best  sodium  or  potassium  cyanide  on 
the  market  for  use  in  the  cyanide  process.  Such  impurities  as 
K2CO3  sulphates  and  sulphides  in  cyanide  cause  serious  losses  in 
the  assay.  In  order  that  the  fusion  may  be  successful,  it  is  essen- 
tial to  follow  directions  closely.  It  is  best  to  use  10  grams  of 
concentrates,  or  an  amount  near  that;  usually  the  amount  of 
concentrates  obtained  from  the  concentration  of  the  ore  approxi- 
mates this  if  the  proper  amount  of  ore  is  chosen  for  concentration. 
Two  grams  of  powdered  cyanide  are  firmly  tamped  into  a  2O-gram 
crucible,  the  concentrates  are  mixed  with  30  grams  more  of 
cyanide,  placed  in  the  crucible,  and  covered  with  5  grams  more. 
The  crucibles  are  placed  in  the  muffle  at  a  full-red  heat  (750°  C), 
and  are  kept  at  this  temperature  for  about  15  to  20  minutes. 
The  charge  will  become  very  liquid,  and  will  be  a  brown-red. 
The  temperature  should  not  be  so  high  as  to  cause  the  cyanide 
to  boil  and  evolve  heavy  fumes.  It  may,  however,  be  kept  too 
low,  in  which  case  the  chemical  reactions  will  not  complete 
themselves  and  the  tin  will  fail  to  collect  into  a  button.  If  the 
concentrates  still  contain  some  foreign  minerals,  the  fusion  takes 
longer  than  20  minutes.  The  crucibles  are  then  withdrawn, 
cooled,  and  the  button  recovered  by  breaking  the  crucible.  There 
will  be  two  distinct  slags,  the  lower  one,  surrounding  the  button, 
usually  light  green,  amorphous  and  subtranslucent,  and  the 
upper  one,  or  fused  cyanide,  opaque,  milk-white  and  coarsely 

1  Hofman,  ibid. 


158  A  MANUAL  OF   FIRE  ASSAYING 

granular,  soluble  in  water.  The  tin  button  should  be  white  and 
soft;  if  not,  it  contains  reduced  impurities. 

The  German  Method.  —  The  German  method  is  based  on  the 
fusion  of  the  cassiterite  concentrates  with  charcoal  and  black 
flux  substitute,  which  has  the  composition,  2  parts  K2CO3,  i  part 
flour.  Five  grams  of  the  concentrates  are  intimately  mixed  with 
i  gram  of  pure  wood  charcoal  and  put  into  a  No.  D  lead  crucible 
or  an  ordinary  2O-gram  crucible.  On  top  of  this  are  placed  15 
grams  of  black  flux  substitute,  with  which  1.25  grams  borax  glass 
have  been  mixed.  Finally  a  pure  salt  cover  is  added,  and  a 
piece  of  charcoal,  the  crucible  covered  with  a  clay  cover,  placed 
in  the  muffle,  and  heated  at  a  moderate  heat  until  boiling  of  the 
charge  has  ceased,  and  then  for  one-half  to  three-quarters  of  an 
hour  more  at  a  white  heat.  The  crucible  is  then  removed  from 
the  muffle,  allowed  to  cool,  and  broken  for  the  tin  button.  This 
should  be  white  and  soft,  as  in  the  cyanide  fusion. 

During  the  fusion,  as  the  temperature  rises,  the  charcoal 
reduces  the  stannic  oxide  to  metallic  tin,  while  any  ferric  oxide 
is  reduced  to  ferrous  oxide,  if  the  heating  is  gradual,  and  is  taken 
up  by  the  slag.  As  the  temperature  rises,  the  flour  in  the  black 
flux  substitute  partially  decomposes,  liberating  carbon  throughout 
the  charge,  which,  as  fusion  takes  place,  prevents  any  stannic 
oxide  not  as  yet  reduced  from  uniting  with  the  alkali  of  the 
flux.  The  slag,  after  cooling,  should  be  crushed  and  panned  for 
any  prills  of  tin  which  have  not  entered  the  button.  These  are 
weighed  and  added  to  the  weight  of  the  button. 

Results  Obtainable.  —  Black  Hills  cassiterite  concentrates, 
roasted,  quenched,  and  treated  with  nitro-hydrochloric  acid  1 

Wet  method  of  Rose-Chauvenet 

with  K2CO3 =  67.84  per  cent.  Sn 

German  method =  67.58  per  cent.  Sn 

Cyanide  method =  67.49  per  cent.  Sn 

Stream  tin  from  Durango,  Mexico,2 

Wet  method  (Rose) =  65.62  per  cent.  Sn 

German  method =  63.92  per  cent.  Sn 

Cyanide  method =  65.19  per  cent.  Sn 

It  is  to  be  noted  that  while  the  dry  methods  approach  very 

1  Hofman,  ibid. 

2  E.  H.  Miller,  "The  Assay  of  Tin  Ores,"  in  "Sch.  Mines  Quart.,"  XIII,  No.  4- 


ASSAY  OF  TIN,   MERCURY,   LEAD,   BISMUTH  159 

closely  to  the  wet  analysis,  which  gives  the  actual  tin  in  the  ore, 
the  dry  assay  results  are  due  more  or  less  to  a  balancing  of  errors. 
Frequently  dry  assays  will  give  higher  results  than  the  analysis; 
this  is  due  usually  to  reduced  iron. 

Of  the  influence  of  foreign  minerals  left  in  the  cassiterite 
concentrates,  quartz  has  the  worst,  causing  heavy  losses.  Feld- 
spar and  tourmaline  have  similar  effect,  but  not  to  so  marked  a 
degree.  Mica  and  garnet  give  high  results,  due  to  the  reduction 
of  iron,  although  tin  is  lost  in  the  slag.  Columbite  acts  in  a 
similar  manner.  With  the  German  method  the  result  is  much 
more  seriously  affected  by  these  impurities  than  with  the  cyanide 
fusion.1 

THE  ASSAY  OF  MERCURY 

Mercury  occurs  in  ores  chiefly  as  cinnabar  (HgS),  and  may 
with  accuracy  be  determined  by  Chism's  method.2  For  low- 
grade  ores,  the  method  is  especially  satisfactory,  and  has  the 
advantage  of  being  rapid  and  short.  It  is  based  on  the  fact 
that  mercury  is  distilled  from  HgS,  etc.,  in  the  presence  of  iron 
filings,  and  can  be  caught  on  silver-foil.  The  difference  in  weight 
between  the  mercury-impregnated  silver-foil  and  the  foil  before 
the  assay  gives  the  mercury.  The  apparatus  required  is  as 
follows : 

1 .  A  small  ring-stand. 

2.  A  fire-clay  annealing  cup  (No.  B  or  C). 

3.  A  piece  of   carefully  annealed    silver-foil    1.5   in.   square, 
which  is  fitted  and  bent  down  to  make  a  reasonably  tight  cover 
for  the  annealing  cup. 

4.  A  flat  silver  or  copper  dish,  holding  20  to  25  c.c.  of  water. 
A  silver  crucible  may  be  used  in  place  of  this. 

.  5.  A  piece  of  asbestos  board,  4  in.  square  and  about  0.20  in. 
thick,  in  the  center  of  which  a  circular  hole  has  been  carefully 
cut,  into  which  the  annealing  cup  will  fit  so  as  to  project  about 
0.5  in.  below  the  bottom  of  the  board. 

.  6.   A  small  alcohol  lamp,  of  about  60  c.c.  capacity. 

7.  A  wash-bottle  with  cold  water,  and  a  glass  tube  for  a 
siphon.  The  silver-foil  is  carefully  fitted  over  the  top  of  the 

1  Hofman,  ibid. 

2  R.  E.  Chism,  in  Trans.  A.  I.  M.  E.,  XXVIII,  p.  444- 


160  A  MANUAL  OF   FIRE  ASSAYING 

annealing  cup,  the  edges  being  bent  down  so  as  to  make  a  close- 
fitting  cover  and  prevent  the  escape  of  mercurial  vapor.  The 
silver  dish  should  be  polished  on  the  bottom,  and  be  in  close 
contact  with  the  foil,  so  that  the  cooling  effect  of  the  water  will 
be  fully  transmitted. 

The  Assay.  —  For  low-grade  ores  from  0.5  to  i  gram  is  taken 
and  mixed  with  from  30  to  50  parts  of  iron  filings.  These  filings 
should  all  pass  a  4O-mesh  screen.  A  select  lot  of  filings  are  best 
digested  with  alcohol  for  some  time  to  remove  oil  and  grease, 
then  heated  in  a  muffle  to  a  dull-red  heat  for  10  minutes,  cooled, 
and  stored  in  a  tight  bottle.  It  is  essential  to  have  the  filings 
free  from  oil  and  grease,  else  this  will  be  deposited  on  the  silver- 
foil  with  the  mercury.  The  amount  of  mercury  in  the  ore  should 
not  be  so  great  as  to  cause  too  heavy  a  coat  on  the  silver-foil. 
For  high-grade  ores,  not  more  than  o.  i  to  0.2  gram  should  be 
used.  Very  small  amounts  of  mercury  can  be  detected  by  this 
method. 

The  ore,  mixed  with  filings,  is  placed  in  the  annealing  cup, 
which  is  set  into  the  asbestos  board  on  the  ring-stand,  the  silver- 
foil  weighed  accurately,  after  igniting,  to  within  o.i  mg.,  and 
fitted  to  the  cup,  and  the  silver  dish,  filled  with  cold  water,  placed 
on  the  foil.  The  alcohol  flame  is  then  allowed  to  play  just  on 
the  bottom  of  the  cup,  but  not  to  spread  around  the  sides.  The 
flame  should  be  about  1.25  in.  high  and  is  best  shielded  by  a 
screen  to  steady  it.  The  bottom  of  the  crucible  should  not 
become  more  than  a  dull  red,  otherwise  mercury  will  escape 
condensation.  The  time  of  heating  should  be  from  10  to  15 
minutes.  It  is  best  to  heat  for  about  10  minutes,  then  cool, 
and  reheat  for  3  to  5  minutes.  Longer  heating  than  this  causes 
loss  of  mercury.  The  degree  and  time  of  heat  are  very  important. 

During  the  heating  the  water  in  the  dish  should  be  replaced 
once  or  twice.  It  can  easily  be  removed  by  a  bent  tube  that 
has  been  filled  with  water,  acting  as  a  siphon.  While  the  warm 
water  is  being  removed,  cold  water  is  added  from  a  wash-bottle. 
After  the  proper  heating,  the  alcohol  lamp  is  removed,  the  assay 
allowed  to  cool  somewhat,  the  silver  dish  removed,  and  the 
silver-foil  with  the  mercury  transferred  by  forceps  to  a  desiccator 
and  then  weighed.  The  difference  in  the  weight  of  the  foil  after 
and  before  the  assay  is  the  weight  of  the  mercury,  from  which 


ASSAY   OF  TIN,   MERCURY,    LEAD,    BISMUTH 


161 


the  percentage  is  calculated.  The  foil  can  be  used  again  after 
driving  off  the  Hg  at  a  red  heat  in  the  muffle,  or  with  a  Bunsen 
burner.  A  piece  of  foil  can  be  used  about  six  times.  It  should 
be  weighed  before  each  assay.  The  method  also  serves  as  a  very 
sensitive  and  easily  applied  qualitative  test  on  ores. 

The  following  figures  will  serve  to  show  the  accuracy  of  the 
method:1 


BY  ELECTROLYSIS  FROM  CYANIDE 

SOLUTION 

Ore  No.  i  I2-37  Per  cent. 

Ore  No.  2  67.26  per  cent. 


BY  CHISM'S  METHOD 


12.44  per  cent. 
67 . 23  per  cent. 


The  accompanying  illustration  (Fig.  44)  shows  the 
employed. 


apparatus 


FIG.  44.  —  APPARATUS  REQUIRED  FOR  THE  MERCURY  ASSAY 


THE  ASSAY  OF  LEAD  ORES 

The  fire  assay  of  lead  ores  will  probably  pass  out  of  use  in 
time,  just  as  the  fire  assay  of  copper  has  done.  At  the  present 
time  it  is  still  largely  used,  although  for  complex  ores  containing 

1  G.  N.  Bachelder,  in  "Sch.  Mines  Quart.,"  XXIII,  p.  98. 


1 62  A  MANUAL  OF   FIRE  ASSAYING 

much  copper  or  bismuth  or  antimony  with  the  lead,  it  is  not  in 
vogue.  It  is,  however,  still  the  criterion  in  the  purchase  of  pure 
sulphides  and  oxidized  lead  ores,  and  also  such  complex  ores  as 
furnished  by  the  Leadville,  Colorado,  district.  Unoxidized  ores 
of  this  type  contain  pyrite,  blende,  galena,  some  little  chalcopyrite 
and  gangue.  Oxidized  ores  contain  cerrusite,  anglesite,  calamine, 
limonite,  etc.,  and  gangue.  The  object  of  the  assay  is  to  bring 
the  lead  of  these  ores  down  into  a  button,  free  from  other  base 
metals,  such  as  Cu,  Zn,  Bi,  Sb,  Fe,  and  free  also  from  S  and  As. 
The  loss  of  lead  by  volatilization  and  slagging  and  the  reduction 
of  base  metals  should  be  kept  to  a  minimum.  As  already  stated, 
this  is  a  difficult  thing  to  do;  so  that  pure  ores  will  invariably 
give  low  results,  and  impure  ones  high. 

There  are  three  methods  of  assay,  differing  in  the  flux  used; 
(i)  the  lead  flux  method;  (2)  the  soda-argol  method;  (3)  the 
cyanide  fusion.  Of  these,  the  lead  flux  method  is  chiefly  used 
throughout  the  West.  The  soda-argol  method  is  a  good  one  on 
ores  not  basic.  The  cyanide  method  is  only  applicable  to  pure 
ores.  With  impure  ores  it  tends  to  reduce  other  base  metals, 
due  to  its  powerful  reducing  action.  Various  mixtures  of  lead 
flux  are  used,  of  which  three  are  made  up  as  follows : 

No.  i  No.  2  No.  3 

4  parts  NaHCOs  2  parts  NaHCO3  6.5  parts  NaHCO3 

4  parts  K2CO3  2  parts  K2CO3  5     parts  K2CO3 

2  parts  flour  i  part  flour  2.5  parts  flour 

i  part  borax  glass  i  part  borax  glass  2.5  parts  borax  glass 

Flux  No.  3  is  probably  the  best  for  most  purposes,  as  deter- 
mined on  a  series  of  ores,  the  results  with  it  being  slightly  higher.1 
For  assay,  10  grams  of  ore  (loo-mesh  fine)  are  mixed  with  30 
grams  of  flux,  placed  in  a  No.  6  or  D  crucible,  or  in  a  2o-gram 
crucible,  covered  with  8  grams  more  of  flux,  and  put  into  the 
muffle  at  a  low  heat,  which  is  then  raised  to  a  light  yellow  (1080° 
C).  The  fusion  should  take  about  30  to  35  minutes.  Nails 
are  added  to  the  charge,  two  tenpenny  nails  for  heavy  sulphides, 
one  for  light  sulphides  or  oxidized  ores.  When  the  charge  is 
taken  from  the  muffle,  the  nails  are  removed  from  the  crucible 
by  a  pair  of  short  hand  tongs,  care  being  taken  to  wash  off  all 

1  McElvenny  and  Izett,  in  "The  Chemical  and  Fire  Methods  of  Determining 
Lead  Ores,"  "Min.  Rep.,"  XLVIII,  p.  26. 


ASSAY   OF  TIN,   MERCURY,   LEAD,    BISMUTH  163 

adhering  lead  globules.  The  crucible  is  then  shaken  and  tapped 
thoroughly,  and  poured.  The  lead  buttons  are  cleaned  by 
hammering  and  weighed.  The  percentage  is  obtained  by  multi- 
plying by  10. 

The  reactions  in  the  crucible  are  as  follows: 

7?bS  +  4K2CO3  =  4?b  +  3(K2S,  PbS)  +  K2SO4  +  4CO2 
K,S,  PbS  +  Fe  =  Pb  +  K2S  +  FeS 
2PbO  +  C  =  2Pb  +  CO, 

The  carbon  liberated,  in  finely  divided  particles  from  the 
flour  on  heating  reduces  any  lead  oxides  or  carbonates  in  the  ore, 
while  the  iron  reduces  lead  from  its  sulphides  and  sulphates. 
The  assay  should  check  (in  triplicate)  within  0.5  per  cent. 

The  soda-argol  method  uses  the  following  flux: 

NaHCO3 6  parts 

Argol    i  part 

For  10  grams  of  ore,  35  grams  of  flux  are  taken,  with  a  light 
flux  cover.  The  fusion  is  performed  as  described  for  the  lead 
flux  method.  The  method  is  good  on  ores  containing  some 
silica,  but  not  on  basic  ores  or  pure  galenas,  as  all  acid  is  lacking 
in  the  flux.  A  borax  glass  cover  is  best  where  the  method  is 
employed  on  basic  ores. 

In  the  cyanide  method,  pure  cyanide  should  be  used,  and  the 
temperature  should  be  kept  much  lower  than  for  the  other  two 
methods.  For  the  regulation  of  temperature,  reference  is  made 
to  the  assay  of  tin  by  the  cyanide  fusion. 

For  the  fusion,  10  grams  of  ore  are  mixed  with  35  grams 
cyanide,  and  a  light  cyanide  cover  used.  Concerning  the  accuracy 
of  the  method  the  following  figures  are  appended:1 

ORE  FIRE  ASSAY  (LEAD  FLUX)    GRAVIMETRIC  (PbSO4) 

Per  cent.  Per  cent. 

1.  Galena  76  78.68 

2.  Galena  37  37-4° 

3.  Cerrusite  9  10.60 

4.  Pyrite,  Sphalerite,  Galena  24.7  18.46 

5.  Galena  and  Stibnite  28.7  27.25 

6.  Cerrusite  37.8  38.60 

1  Determination  of  Lead  in  Ores,  I.  T.  Bull,  S.  of  M.  Quart.,  Vol.  XXII,  p.  348. 


[64  A    MANUAL   OF    FIRE   ASSAYING 


THE  ASSAY  OF  ANTIMONY  AND  BISMUTH  ORES 

For  accurate  and  satisfactory  determinations  on  these  ores, 
wet  methods  must  be  resorted  to.  Antimony  occurs  chiefly  as 
the  sulphide  stibnite,  although  the  oxides  and  some  native  metal 
are  found  as  ore.  Bismuth  as  an  ore  occurs  chiefly  as  the  native 
metal,  but  is  found  also  in  combination  with  oxygen,  sulphur, 
etc.  For  the  assay,  the  following  charge  is  best : 

Ore 10  grams 

Cyanide   40  to  50  grams 

Cover  of  cyanide. 

Fuse  at  a  full-red  heat,  as  given  for  tin,  for  30  minutes.  The 
resultant  buttons  are  brittle  and  cannot  be  hammered. 


INDEX   TO    AUTHORS 

NAME  PAGE 

Allen,  E.  T.,  see  Hillebrand  and  Allen 

Ames  and  Bliss    39 

Bailar,  J.  C 79 

Balling,  C 60 

Batchelder,  G.  N 161 

Bettel,  W 68 

Bowman,  F.  C 4 

Bowman  and  Mason 120 

Brunton,  W.  D 29 

Bull,  I.  T 163 

Carter,  H.  L 114 

Chism,  R.  E 159 

Crawford,  C.  H t 101,  113,  129 

Day  and  Shepard 22 

Eager  and  Welsh 120,  122 

Edmands,  H.  K 70 

Fulton,  C.  H 101,  105,  113,  120,  128,  129,  130 

Godshall,  L.  D 121,  122 

Gottschalk,  V.  H 34 

Hempel,  A 23 

Hemtz,  F 67 

Hillebrand,  W.  F.,  and  Allen     100,  104,  105,  125,  126,  128,  132,  133 

Hofman,  H 90,  156,  157,  159 

Howe,  H.  M 80 

Izett  and  McElvenny    162 

Janin,  Jr.,  Louis   4 

Kaufman,  W.  H 120,  122 

Keller,  Edward 1 2 ,  32 ,  82 

Kemp,  J.  F 147 

Kerl,  B 67 

Kitto,  Wm 114 

Koenig,  G.  A i 

Lay,  F 113 

Lodge,  R.  W 78,  91,  121,  123,  128,  129,  149 

Lenher,  Viet 133 

Mason  and  Bowman 120 

Miller,  E.  H 45,  49,  90,  93,  120,  129,  130,  147,  148,  153,  158 

165 


166  INDEX    TO   AUTHORS 

NAME  PAGE 

Nutter,  E.  H 4 

Ostwald,  W 39 

Pack,  J.  W 142,  144 

Perkins,  W.  G 90, 109 

Roberts,  G.  M 32 

Roberts-Austin ^ . .  .  : 22 

Rose,  T.  K 70,  72,  76,  81,  96,  112,  125,  126,  127,  132,  133,  143,  148,  149 

Sander,  K 76,  114 

Sharwood,  W.  J 148,  149 

Shepard  and  Day 22 

Schorlemmer  and  Roscoe 55 

Smith,  E 77 

Smith,  E.  A 114,  115 

Smith,  F.  C 79,  105 

Schiffner,  M 148 

Sulman,  H.  L 115 

Thompson,  T 147,  148 

Vail,  W.  G 116 

Van  Liew,  R.  W 99 

Van  Nuys,  C.  C 123 

Vogt,  J.  H.  L 58 

Warwick,  A.  W 33 

West,  E.  E 59 

White  and  Taylor   80 

Williams,  D.  J 112 

Woodward,  E.  C 103,  128 


INDEX 


PAGE 

Absorption  of  precious  metals  by  cupels 69 

Accuracy  of  the  gold-silver  assay    134 

Action    of  acids  on  silver-platinum  alloys 148 

arsenic  and  antimony  in  the  roasting  of  ores 87 

soda  in  forming  sulphates     44,  85 

Addition  of  silver  to  the  assay 26 

Allotropic  gold .' 83 

Alternate  shovel  method  of  sampling 30 

Alumina,  melting-point 23 

Amount  of  lead  reduced  by  carbonaceous  reducing  agents 45,  47 

sulphides    45, 47 

tellurium  present  in  telluride  ores    105 

Analysis  of  fire-clay  for  crucibles 17 

hematite    65 

lead-antimonial  ores 65 

limestone 65 

mattes in 

silicious  ores 64 

Annealing  parted  gold 83 

Antimonial  ores,  assay  of  for  antimony 164 

gold 114 

difficulties  of  assay 116 

Apparatus  required  for  wet  silver  bullion  assay 139 

Appearance  of  cupeled  bead  in  the  presence  of  platinum,  etc 148 

Applicability  of  the  roasting  method  of  assay 88,  93 

Argol 23 

reducing  power  of   45  >  47 

reduction  of  lead  by    43 

temperature  of  reaction  with  niter    50 

Arsenical  ores,  difficulties  of  assay 116 

methods  of  assay 115 

Assay  balance 34 

adjustments  for 37 

calculations  for   36,  37,  38 

construction  of   34 

sensibility  of. 36 

167 


168  INDEX 

PAGE 

Assay  of  Black  Hills  Cambrian  ores 105 

results  obtained  by  different  fluxes  .  .  106,  107 

cupels 108,  1 19 

ores  containing  impurities 84 

ores  for  base  metal 155 

reagents    24 

slags 56 

Assaying,  definition  of 20 

for  gold  and  silver 20 

B 

Balance,  sensibility  of 36 

zero-point  of 37 

Balance-arms,  inequality  of .- .  38 

Balling's  table  for  the  calculation  of  slags 60 

Bases  in  assay  slags 57,  58,  60 

Behavior  of  lead  sulphate  in  roasting 88 

silver  in  the  roasting  of  ores 87 

tellurium  in  the  crucible  assay 104 

Bismuth  ores,  assay  for  bismuth 164 

Black  flux  23 

substitute 23 

Boiling-point  of  zinc 112 

Bone,  composition  of 67 

Bone-ash,   composition  of 67 

melting-point  of 68 

screen  analysis  of 68 

Borax  glass 22 

melting-point  of 22 

Bullion  assay 135 

Bullions,  classification  of    135 

C 

Calculation  of  assay  slags 61,  62,  63 

Calculations  in  the  wet  silver  bullion  assay 141,  142 

Capacity  of  assay  furnaces 3,  5,  10 

Carey  gasolene  burner    9 

Charcoal,  carbon 23 

reducing  power  of 44,  47 

Check  assays  for  bullion 137 

Chemical  and  physical  properties  of  zinc    112 

Classification  of  bullions    135 

Color  of  slags 66 

scale  of  temperature    80 

Combination  method  of  assav  •  9& 


INDEX  169 

PAGE 

Combination  method  of  assay  for  cyanide  precipitates 101 

mattes    99 

general  precautions  to  be  observed  in  method     100 
Van  Liew's  modification  for  blister  copper  .  .       98 

Comparison  of  different  crucible  methods  of  assay 93 

results,  crucible  method,  between  shipper  and  refiner no 

and  scorification  method  on  matte   no 

Composition  of  Black  Hills  Cambrian  ores    105 

Conditions  to  be  observed  in  the  crucible  assay 54,  55,  56 

Coning  and  quartering 29 

Control  assays 32 

of  heat  in  roasting 86 

Copper,  influence  on  cupellation 79 

Copper-bearing  material,  assay  of  by  combination  method 98,  99 

crucible  method    109 

scorification 108 

losses  in  the  assay  of 130 

Correction  for  gold  and  silver  in  the  assay  of  copper  mattes    109 

Cost  of  fuel  in  assaying    4,5,10 

Cripple  Creek  flux  for  telluride  ores    103 

Critical  temperature  for  reaction  between  carbon  and  litharge 55 

niter  and  charcoal 50 

silica 50 

sulphides 50 

Crucible  assay 54 

for  copper-bearing  material   109 

influence  of  fineness  of  ore  on 54 

mode  of  occurrence  of  precious  metals 54,  55 

physical  and  chemical  properties  of  the  slag. ...  .5,  54. 

Crucibles,  capacity  of    17 

Cupel  charging  device 15 

Cupel-machines    69 

Cupel-trays 16 

Cupellation 67,  70 

gold  losses  in 123,  125 

influence  of  antimony  on 79 

copper  on    78 

other  impurities  on   77 

tellurium  on 79 

loss  for  gold-silver  alloys 125,  126 

method  of  assay  for  silver  bullion 136 

proper  temperature  for 71 

silver  losses  in 121, 122 

Cupels,  assay  of 108, 119 

manufacture  of 69 

Curves  showing  silver  losses  in  cupellation 124 


170  INDEX 

PAGE 

Cyanide  fusion  for  gold-silver  ores 92 

defects  of  method 93 

temperature  required  for  assay 92 

typical  charge 92 

method  of  assay  for  tin  ores    157 

precipitates,  combination  method  of  assay 101 

crucible  method  of  assay 113 

D 

Definition  of  assaying 20 

Difficulties  of  assay  of  arsenical  and  antimonial  ores 115,  116 

Dimensions  of  furnaces .     2 ,  3 

E 

Effect  of  different  reagents  in  the  assay  of  impure  ores 85 

impurities  on  the  crucible  assay 84,  91,  92 

Equations  of  the  balance 36,  37,  38 

Errors  in  the  assay  for  gold  and  silver 120, 132, 133 

of  lead  ores 162 

tin  ores 155, 156 

weighing 133 

Example  of  the  calculation  of  assay  slags 61,  62,  63 

Excess  litharge  method  of  assay 90 

charge  for  blister  copper    in,  112 

high-grade  mattes in 

low-grade  mattes     in 

temperature  required  for  fusion 90 

typical  charge  for 90 

F 

Ferric  oxide 23 

Fineness  of  bullion    135 

Fire-clay  for  crucibles  and  scorifiers 17 

space  in  assay  furnaces 2 

Flasks  for  parting    . 83 

Flour,  reducing  power  of 23, 45, 47 

Flue  area  in  coal  muffle-furnaces 2 

Fluorspar,  in  the  assay    22 

reassay  of  cupels    119 

Formation  of  lead  sulphate  in  scorification 97 

matte  in  the  crucible  assay 84 

Freezing-point  curve  of  lead-copper  alloys 74 

lead-silver  alloys    74 

Fuel  consumption  in  assay  furnaces 4,  5,  10 

Fuels  for  heating  assay  furnaces i 


INDEX  ,7, 

PAGE 

Fuels  for  heating  assay  furnaces,  coal x>  4 

coke   •• i,S 

crude  oil    !  ?  4 

gas    10 

gasolene i ,  6, 8 

wood !  f  4 

Furnaces  used  in  assaying   ! 

muffle-furnace i 

pot  furnace i 

Furnace  tools 13 

crucible  and  scorifier  tongs    13 

cupel  charging  device    75 

tongs    13 

multiple  scorifier  tongs 14 

G 

Gas  furnaces  for  assaying    10 

Gasolene  furnaces  for  assaying 6,  7 

consumption  of  gasolene  in 10 

cost  for  fuel  in 10 

temperature  attainable  in    9 

Gay-Lussac  method  of  assay  for  silver  bullion    138 

General  formula  for  assay  slags    64 

German  method  of  assay  for  tin  ores    158 

Gold  bullion,  assay  of 142 

final  assay 143 

preliminary  assay 142 

sampling    142 

H 

Hematite 10 

Hillebrand  and  Allen  charge  for  telluride  ores 104 


I 

Impurities  in  ores,  classification  of 85 

Influence  of  cupel  material  on  cupel  absorption 122 

foreign  minerals  on  the  tin  assay 156,  159 

size  of  lead  button  on  cupel  loss    122, 124 

Inquartation 81 , 82, 142 

Indium,  assay  of 14? 

action  of  nitric  acid  on 148 

sulphuric  acid  on    150 

behavior  of  in  the  assay 148 

in  platinum  nuggets  from  the  Urals 14? 


172  INDEX 

PAGE 

Iron  nail  method  of  assay gi 

comparison  of  varying  charges    91,  92 

effect  of  silicate  degree  of  slag  on  assay 85,  91 

reactions  taking  place  in 91 

temperature  required 94 

typical  charge    91 

J 

Jamesonite,  method  of  assay  for  gold  and  silver 115 

Jones  sampler 32 

L 

Lead    23 

bullion,  assay  of      136 

sampling  of    32 

flux 23, 162 

method  of  assay  for  lead  ores 162 

melting-point  of 23 

ores,  assay  of 161 

lead  flux  method  of  assay  for    162 

reactions  in  the  assay  of 163 

soda  and  argol  method  of  assay  for 162 

ratio  in  the  cupellation  of  gold  bullion    144 

silver  bullion    137 

silicates,  reduction  of  lead  from 46 

Lime,  physical  properties 22 

Litharge    21 

amount  of  silver  in     27 

assay  of  to  determine  gold  and  silver 25 

melting-point    21 

reactions  with  silica 21 

reduction  of  lead  from .  .  . 21 ,  43,  44,  45 

Loss  of  gold  and  silver  in  the  cupellation  of  gold-silver  alloys 125,  126 

by  cupel  absorption 125,  127 

volatilization  during  cupellation 125,  127 

silver  in  roasting  ores    87 

Losses  in  the  crucible  fusion     128,  130 

cupellation   of  gold  and  silver 120 

pure  gold 123 

silver    121,122 

scorification  assay 129, 130, 131 

of  gold  and  silver  in  the  assay  of  carbonate  ores  and  silver  sulphides     130 

copper-bearing  material 130 

telluride  ores .      128 

zinciferous  material 129 


INDEX  i?3 

PAGE 

M 

Magnesia 22 

Matte,  copper,  assay  of 98,  99,  108,  109 

formation  of   84 

Mercury,  assay  of 159 

apparatus  required  for 159 

results  obtainable  in 161 

in  the  silver  bullion  assay    141 

Methods  of  assay  for  impure  ores    86 

Miller's   oxide  slag  method  of  assay    89 

temperature  required  for  fusion    89 

typical  charge 89 

Millieme,  definition  of 135 

Moisture  determination  for  ores    31 

in  assay  samples 31 

Molds 16 

Muffle -furnaces,  advantages  of i 

types  of 2,3 

Muffles 5 

Muffle  supports 3 

Multiple  scorifier  tongs 12,13 

N 

Nature  of  bases  in  assay  slags 57,  58,  60 

Niter 24 

charge  to  determine  oxidizing  power 52 

iron  method  of  assay 92 

method  of  assay 88 

for  sulphides 116 

oxidizing  power    49 

reaction  between  niter  and  carbon    48 

lead 48 

silica 50 

sulphides. 49>5J 

Nitric  acid,  action  on  silver-platinum  alloys 148 

strength  used  in  parting 80,  81 

O 

Occluded  gases  in  parted  gold 133 

Order  of  oxidation  of  metals 96 

Ores  containing  free  gold,  assay  of 117 

metallic  scales,  assay  of    117 

Oxidation    43,47 

reactions    43 

carbon  by  niter 48 


174  INDEX 

PAGE 

Oxidation  reactions,  lead  by  niter 48 

sulphides  by  niter 49,  51 

Oxidizing  power  in  ores   53 

of  niter 49 

P 

Palladium,  solubility  of 150 

Parting,  apparatus  for 83 

bath   83 

cups  and  flasks 83 

gold  bullion  assays 143 

gold  from  silver   81 

ratio  of  gold  to  silver  necessary 81,  142 

strength  of  acid  required 81 ,  82,  143 

temperature  of  acid  necessary 82,  143 

washing  the  parted  gold 82 

Perkins'  excess-litharge  method    90 

Platinum  alloys,  action  of  nitric  acid  on 149 

sulphuric  acid  on 149,  150 

methods  of  assay 150, 151 

in  cupellation    148 

nuggets  from  the  Urals,  composition  of 147 

ores,  methods  of  assay   150,  151,  152,  153 

Plumbago  crucibles,  assay  of 113 

Potassium  carbonate 22 

cyanide 24 

reaction  of  with  galena 24 

litharge 24 

nitrate,  see  niter 24 

Proof  gold,  preparation  of 145 

silver,  preparation  of 146 

Pyrite,  assay  of 1 16 

reaction  with  litharge    45 

in  presence  of  soda    44 

niter 51 

silicates  of  lead    85 

reducing  power 45  >  47 

R 

Reactions  in  assay  of  lead  ores 163 

the  roasting  of  ores 86,  87,  88 

with  lead  silicates  and  carbon     46 

sulphides 85 

litharge  and  argol 43 

carbon    44 

pyrite 44>  45 


INDEX  ,75 

PAGE 

Reactions  with  niter  and  carbon  , 


lead  ..................................  ......       4g 

silica  .......................................       r0 

sulphides  ....................................  49)  5I 

Reagents,  list  of  ....................................................       2I 

Reducing  agents,  see  Tables  I  and  II  .  ................................  45>  47 

Reduction  of  lead  from  litharge  .  .......................  ...........  4-^  44j  etc. 

Requirements  for  parting  .........  ...................................  gr   g2 

Residues  from  zinc  distillation,  assay  of  ................................      II4 

Results  obtainable  in  the  lead  assay    ..................................     ^3 

mercury  assay  ...............................      j6i 

tin  assay  ....................................      I58 

Retention  of  copper  by  silver  beads  ...................................      I08 

lead  in  cupeled  beads  ....................................      132 

silver  by  parted  gold    ....................................     I^2 

Riders  .............................................................  38,  40 

Riffle  sampler  ......................................................        32 

Roasting  method  for  stibnite  .........................................      u^ 

of  assay  .......................................  86 


S 

Salt 24 

" Salting"   of  fluxes   24 

samples    32 

Sampling 28 

alternate  shovel  method    30 

at  plant  of  Standard  Smelting  Co 31 

by  hand 28 

machine  28 

coning  and  quartering 28,  29,  30 

gold  and  silver  bullion    142 

lead  and  copper  bullion 32,  33 

methods  of    28 

principles  of 28,  30 

required  size  of  sample 29 

Scorification   method   of  assay 94 

addition  of  borax  glass  and  silica  to  assay.  .95,98 

amount  of  ore  taken  for  assay 95 

test  lead  used    95 

applicability  of 97 

applied  to  copper-bearing  material 107, 1 08 

effect  of  bases  in  ores  on 95 

copper  and  other  impurities  on    ...       96 

nature  of  slag  in 94,  95,  97 

slag  loss  in 129,  130,  131 


176  INDEX 

PAGE 

Scorifiers,  capacity  of 18,  94 

Selenium,  effect  on  cupellation    79 

Silica,  melting-point  of 32 

properties  of 32 

Silicate-borates 55,  57,  58 

Silicate  degree  of  assay  slags 57 

chemical  classification    57 

metallurgical  classification 57 

Silicates  of  lead 46,  85 

Silver  bullion    135 

assay  of  by  cupellation   136 

wet  method    138 

sampling  of 142 

standardizing  solutions  for  assay 139 

in  litharge    27 

Slags,  color  of 66 

nature  of  bases  in 57,  58,  60 

oxide 89,  94 

reassay  of    117 

silicate  degree  of 57 

solvent  power  for  matte :  ... 85,  91 

Soda-argol  method  of  assay  for  lead  ores   162 

Sodium  carbonate    21 

melting-point 22 

reactions  with  silica 21 

sulphate,  formation  of   44 

melting-point  of 45 

Solution  of  gold  by  nitric  acid 133 

Solvent  power  of  litharge  for  other  bases    90 

slags  for  mattes 85,  91 

Special  methods  of  assay   103 

"Spitting"  during  cupellation 70 

" Sprouting"  during  cupellation 71,  138 

Standardizing  solutions  for  the  silver  bullion  assay 139 

Stibnite,  assay  for  gold  of    115 

Sugar,  reducing  power  of 45,  47 

Sulphides,  assay  of 116 

Sulphuric  acid,  action  on  platinum-silver  alloys 149 

used  in  parting  81,  149,  151 

T 

Table  of  analysis  of  hematite    66 

lead-antimonial  ores 65 

limestone 66 

silicious  ores 64 

assay  slags   59 


INDEX  177 

PAGE 

Table  of  calculation  of  slags 60 

comparison  of  crucible  and  scorification  method  on  copper-bearing 

material no 

copper  influence  in  cupellation 78 

influence  of  impurities  on  cupellation 77 

lead  ratio  in  cupellation  of  gold  bullion   145 

silver  bullion    137 

litharge  required  to  dissolve  metallic  oxides 90 

loss  in  assay  of  cupriferous  material 130 

high-grade  carbonate  and  silver  sulphide  ores  ....  130 

telluride  ores    128 

zinciferous  material 129 

cupellation  of  gold-silver  alloys    126 

pure  gold 123,  125,  126 

silver 121,  122 

of  silver  in  cyanide  fusion  for  gold  and  silver 93 

oxidizing  power  of  niter    49 

quantity  of  tellurium  in  ores 105 

reducing  power  of  agents 45  >  47 

silicate  degree  of  slags 57 

tellurium  influence  on  cupellation    79 

Telluride  ores,  assay  of 103 

by  combination  method    105 

Cripple  Creek  flux    103 

Hillebrand  and  Allen  charge 104 

fineness  of  crushing  required 104 

losses  in  the  assay  of 128 

quantity  of  tellurium  present 105 

temperature  required  for  fusion    104 

Tellurium,  behavior  in  the  assay    104 

effect  on  cupellation    79 

Temperature,  color  scale  of 80 

critical  for  reaction  between  carbon  and  litharge 55 

niter  and  charcoal 50 

silica 50 

sulphide 50 

influence  on  loss  of  gold  in  cupellation    123 

silver  in  cupellation    121 

in  roasting 86,  87 

of  cupellation   71 ,  75 

in  presence  of  platinum    148,  I51 

Test  lead 23 

assay  of  to  determine  gold  and  silver 26 

Tin  assay,  results  obtainable 158 

ores,  cyanide  fusion  for 157 

difficulties  of  assay IS6 


178  INDEX 

PAGE 

Tin  ores,  German  method  of  assay 155 

preparation  of  ore  for  assay 157 

Tools  used  in  assaying 13,  14,  15 

U 

Umpire  assays    32 

V 

Van  Liew's  combination  method  of  assay  for  blister  copper 99 

W 

Weighing  on  the  assay  balance 34,  37,  39 

Weights,  assay -ton  system 41 

gram 40 

milligram 40 

standardization  of    41 

Z 

Zinc,  affinity  of  for  precious  metals   112 

boiling-point 112 

oxide 112 

vaporization  of 112 

volatilization  of 112 

Zinciferous  material 112 

assay  of,  combination  method 101 

crucible  methods 113 

scorification   112 

losses  in  the  assay  of 129 


YC  33872 


